“Adaptation to definite life-purposes,” as Sachs terms it (_Physiology

of Plants_, 1887, p. 122), is seen everywhere; but it is adaptation

restricted to the organism’s conditions of existence. It is not

conceivable, as I have said, that an organism can be adapted to

conditions outside its environment. If there is such a seeming

adaptation, it is but a blind result, the accidental outcome of

collision or contact between two sets of conditions. If we represent a

number of these sets of conditions by several circles gradually

increasing in size until they encroach on each other, we find that the

circles lose their form and acquire a polygonal shape. All characters

seemingly connected with modes of dispersal have only this indirect

relation to such agencies; and their utility in these respects is an

accident in the plant’s life. They have not been acquired in connection

with the dispersing medium, but are the products of the laws of growth

and heredity, guided by a determining agency, and acting within the

organism’s conditions of existence. It is within these narrow limits

that all evident adaptations lie. In matters outside the conditions of

the development of seeds and fruits, the evolutionary or determining

principle “lets them go.” Detached from the plant, they come in contact

with conditions for which they were never created. The predominant power

in Nature, that brings to a successful issue the development of an

organism, has its limitations, and this is one of them, the evolutionary

or determining influence being ever checked and hampered by the laws of

the inorganic world.

I can only refer briefly to some of the reasons that have led me to

apply this view of the duality of forces in Nature to the subject of

plant-dispersal. The principles of evolution and adaptation rule the

world except in matters of dispersal. Take, for instance, the fleshy

fruits which the gardener often makes more attractive to birds than they

are in the wild condition. The result is certainly to increase their

facilities for dispersal by birds; but such a result was as little

intended by man as it was by Nature when species of Cornus, Ficus,

Prunus, Viburnum, and other genera matured their drupes, berries, and

fleshy fruits in the Cretaceous epoch.

Children are now taught in several excellent little books on

“Nature-Study” that fleshy fruits are specially adapted to be eaten by

animals to secure the distribution of the seeds. We read in one book

that plants produce these fruits “on purpose to be eaten,” in another

that they are “intended to be eaten,” and in a third that the

seed-coverings are adaptations, all with the ulterior object of

distribution by frugivorous animals. I must be pardoned if I venture to

express my dissent from these statements, more especially since they are

made by authors from whom it might be thought almost impertinent for me

to differ. Yet authority can be claimed for holding the opposite view.

When the botanist speaks of “useless secretions” in a plant, he is

alluding amongst other things to the sugar and organic acids of fruits.

“How and why all these substances originate is,” as Professor Sachs

observed in the work before quoted, “not known.” It is, however,

suggested by Dr. Kerner, in his _Natural History of Plants_ (Engl. edit.

i, 460-462), that such secretions, though useless to plants, may exist

for the purpose of alluring animals to assist in seed-dispersal. There

are some botanists, it may be remarked, that would reject such a view of

the nature of fruits. Dr. Stapf in his memoir on the flora of Kinabalu

observes in this connection that the fact that a fruit is fleshy and

attractive to birds is “no proof that it is really devoured by them, and

still less that it is dispersed by them.” Neither in fleshy fruits, nor

in minute seeds, nor in seeds capable of being transported by the wind

does he regard the general object of the particular character as

primarily to act as a means of dispersion.

The same plea is made for the mucosity of seeds like those of Capsella

and Plantago (see Note 43), or for the “stickiness” of other seeds and

fruits like those of Pisonia, qualities that favour adherence to passing

objects. This is the reason, we are told, why seeds are “sticky.” Such

secretions I infer are often materials lost to the plant; and being in

that sense excretory we are not called on to supply a use for them. They

can, therefore, not be regarded as having any teleological significance,

since adaptation arises only from the requirements of the plant’s

conditions of existence. If they are serviceable in assisting the

distribution of seeds, such an event can only be described as an

accident in the plant’s life arising from chance contact with another

environment.

The appendages of seeds and fruits, such as hooks and hairs, that render

them liable to adhere to fur or feathers, are also regarded as special

adaptations to this end. Without entering into the physiological

significance of hairs and prickles generally, concerning which, as many

of my readers will know, much might be said not in favour of such a

view, I would refer to cases like that of Cæsalpinia Bonducella, where

the large prickly pods could not possibly be intended to aid the plant’s

dispersal, whilst the leaf-branches are also prickly, and the seeds are

well known to be distributed by the currents. There are other cases like

that of Bidens cernua where the achenes, by reason of their barbed

bristles, and on account of a layer of “buoyant tissue” in the

fruit-coats, are dispersed both by birds and by water. We may fitly ask

to which capacity the theory of adaptation should be applied. Spiny

fruits may be sometimes so large, as in the instance of Trapa natans,

that the question of adaptation to dispersal cannot be raised.

The great variety of the modes of dispersal of seeds is in itself an

indication that the dispersing agencies avail themselves in a hap-hazard

fashion of characters and capacities that have been developed in other

connections. Seeds and fruits, having developed certain characters under

a particular set of life-conditions, on being detached from the parent

plant are brought into contact with conditions quite outside their

original environment. Qualities and capacities are then brought into

play which have no connection with the life-history of the plant. The

care with which the mother plant guards the maturing seeds, and the

protection of the environment, are at a certain period withdrawn, and

the seeds are left to take their chance under strange conditions. It

would be idle to see anything purposeful in the waste that results.

Rather we would see in it the effect of one of the numerous limitations

of the determining or evolutionary power in Nature. Such a power has to

adapt its workings to the laws of the physical world, checked here,

frustrated there, at times, as in this particular case, losing all

control, but in the end prevailing.

My general position may be thus summarised. As concerning the

distribution of fruits and seeds, the dispersing agencies take advantage

of characters and capacities that were never intended for them,

characters and qualities indeed that are often only brought out in

relation to another environment. Thus no question of adaptation as

regards means of dispersal can arise, since such capacities for

dispersal have no connection with the plant’s life-history. That seeds

are dispersed at all is a blind result of the ever-continued struggle

between the opposing forces of evolution and adaptation; that is to say,

between the determining power that lies behind organic life and the

physical conditions to which it has to adapt its ends.

CHAPTER XII

THE CAUSES OF THE BUOYANCY OF SEEDS AND FRUITS OF LITTORAL PLANTS WITH

ESPECIAL REFERENCE TO THOSE OF THE PACIFIC ISLANDS

The classification of buoyant seeds and fruits.—The first group, where

the cavity of the seed or seedvessel is incompletely filled.—The

second group, where the kernel is buoyant.—The third group, where

there is air-bearing tissue in the seed-tests or fruit-coats.—The

buoyant seeds and seedvessels of the littoral plants of the British

flora.—Summary.

IN the following pages I have adopted in its main features the

classification of buoyant seeds and fruits employed by Professor

Schimper in his work on the strand-flora of the Indo-Malayan region. The

causes of buoyancy, as he points out, are very various, but they can be

arranged in a few categories; each category, however, usually admitting

great variety within its limits. It is this want of uniformity that

first attracts our attention when we come to study the structure of

seeds and fruits from the standpoint of their buoyancy. Whilst in the

Pacific I went over most of the field traversed by Professor Schimper in

Malaya (the majority of littoral plants of these regions being common to

both), and as a result I have added not a few plants to his original

groups.

It will be seen from the following synopsis that there are three

principal groups. The first group includes those seeds and fruits where

the buoyancy is derived from unfilled space in the seed or fruit cavity.

The second group comprises those seeds or fruits where the floating

power is due to the buoyant kernel or nucleus. The third group includes

those where the buoyancy arises from the existence of air-bearing tissue

in the coverings of the seed or fruit.

The first two groups I will term the mechanical or non-adaptive groups,

not only on account of the structure inducing the buoyancy, but because,

as Professor Schimper remarks, the same structure often occurs with

inland fruits and seeds possessing little or no floating power. In many

of these cases, as he points out, the question of adaptation to

dispersal by ocean currents cannot, therefore, be raised. The third

group may be named the adaptation group, because it is on these examples

of buoyant seeds and fruits that this investigator chiefly based his

contention that in the main the structures concerned with buoyancy

represent adaptations to dispersal by currents effected through the

agency of Natural Selection. It is accordingly to this group that

Professor Schimper especially directed his attention, and the result of

his observations made in the home of the plants and of his

investigations in the laboratory has been the elucidation of many

difficult points in the structure of their fruits and seeds. To the two

“mechanical” groups he did not pay the same attention; and as their

examination came more within the limits of my own capacity as an

inquirer I have worked them out with some detail, the subdivisions of

the first group being my own as well as much of the material.

_Synopsis of the buoyant fruits and seeds of littoral plants of the

tropical Pacific classified according to the cause of buoyancy._ (The

authorities are indicated by the initial letter, S = Schimper, G =

Guppy. Details are given under some of the species in latter part of

volume.)

FIRST GROUP.—The floating power is derived from unoccupied space in the

cavity of the seed or fruit, no part of the seed or fruit as a rule

possessing independent floating power.

SUB-GROUP I., where the seed is concerned.

SECTION I. The seeds have little or no albumen, and neither the tests

nor the seed-contents have any buoyancy. The cotyledons are generally

large, foliaceous, and crumpled or folded, or otherwise arranged, so

that the seed-cavity is incompletely filled.

S. G. Hibiscus tiliaceus.

G. Hibiscus diversifolius.

S. G. Thespesia populnea.

S. Suriana maritima.

G. Kleinhovia hospita, _variable_.

S. G. Colubrina asiatica.

S. Dodonæa viscosa.

G. Argyreia tiliæfolia, _variable_.

G. Ipomœa bona nox, _variable_.

G. Ipomœa glaberrima, Boj.

S. G. Ipomœa grandiflora.

S. G. Ipomœa pes capræ.

G. Ipomœa turpethum, _variable_.

G. Cassytha filiformis.

S. Euphorbia atoto.

_Notes._—The species marked “variable” have seeds that sometimes sink

and sometimes float. With the exception of Kleinhovia they are only at

times littoral in station.

The plants of the British flora are represented by Convolvulus

soldanella and C. sepium, the last being “variable” and not a littoral

species.

SECTION II. All the seeds belong to the Leguminosæ. Neither the tests

nor the seed-contents have any buoyancy, the floating power arising

from a large central cavity produced by the bending outward of the

cotyledons during the final shrinking stage of the maturation of the

seed.

S. Mucuna (generically).

G. Mucuna urens D.C. (Hawaii).

G. Mucuna, species of.

S. G. Vigna lutea.

S. G. Cæsalpinia bonducella.

G. Cæsalpinia bonduc.

G. Entada scandens.

SUB-GROUP II., where the fruit is concerned.

SECTION III. The seed only partially fills the fruit-cavity, and as a

rule is not buoyant. The fruit shell, usually woody, may be also

buoyant.

S. G. Heritiera littoralis.

G. Smythea pacifica.

G. Dalbergia monosperma.

S. G. Derris uliginosa.

S. G. Pongamia glabra.

G. Desmodium umbellatum.

G. Gyrocarpus jacquini.

SECTION IV. The floating power is derived from empty seed-cavities,

where owing to abortion of the ovule or some similar cause the seed

is not developed.

S. G. Morinda citrifolia.

G. Premna tahitensis.

_Note._—Professor Schimper, in the case of Morinda citrifolia, holds the

view that we have here a special adaptation to dispersal by currents.

SECOND GROUP.—Here the floating power is due mainly or entirely to

buoyant kernels. In the case of seeds the tests are non-buoyant; but

with “stones” the floating capacity may be aided by a layer of

air-bearing tissue inside the shell.

SECTION I. Non-Leguminous.

S. G. Ximenia americana (drupe).

S. G. Calophyllum inophyllum (drupe).

_Note._—Professor Schimper would place these two plants in the second

section of the third group on account of the layer of air-bearing tissue

inside the shell of the “stone”; but they are assigned to this section,

since the floating power is mainly due to the buoyant kernel.

Arenaria (Honckeneya) peploides, a British beach plant, belongs here.

SECTION II. Leguminous seeds.

G. Dioclea.

G. Strongylodon lucidum.

S. Canavalia (generic).

G. Canavalia sericea.

S. G. Canavalia obtusifolia.

S. Erythrina (generic).

S. G. Erythrina indica.

P. Erythrina ovalifolia (Penzig).

S. G. Sophora tomentosa.

G. Afzelia bijuga.

G. Lathyrus?

THIRD GROUP.—The floating power is due to the presence of air-bearing

tissue in the seed-tests or fruit-coats.

SECTION I. The buoyant tissue occurs at the outside or forms the

periphery of the seed or fruit. Unless otherwise indicated the fruit

is implied in the list below.

S. G. Carapa moluccensis (seed).

S. G. Carapa obovata (seed).

G. Inocarpus edulis.

G. Serianthes myriadenia.

G. Parinarium laurinum.

S. G. Barringtonia speciosa.

G. Barringtonia racemosa.

S. G. Pemphis acidula (seed).

S. Terminalia (generic).

S. G. Terminalia katappa.

G. Terminalia litorea.

S. Lumnitzera (generic).

S. G. Lumnitzera coccinea.

S. G. Guettarda speciosa.

G. Wedelia strigulosa.

S. G. Scævola Kœnigii.

S. G. Cerbera Odollam.

G. Ochrosia parviflora.

S. G. Cordia subcordata.

S. G. Tournefortia argentea.

S. G. Clerodendron inerme.

G. Vitex trifolia.

G. Vitex trifolia, var. unifoliolata.

G. Tacca pinnatifida (seed).

S. Nipa fruticans.

S. Cocos nucifera.

G. Scirpodendron costatum.

_Additions of shore-plants from Malaya and tropical America mostly given

in Schimper’s work on the Indo-Malayan strand-flora._

S. Cynometra cauliflora.

S. Conocarpus erectus.

S. G. Laguncularia racemosa.

S. Lumnitzera racemosa.

S. Sonneratia (seed).

S. Barringtonia excelsa.

S. Scyphiphora hydrophyllacea.

S. Wollastonia glabrata.

G. Hippomane mancinella.

_Note._—Here belong a species of Vitex, probably V. agnus castus, the

fruits of which occur in the stranded drift of the Sicilian beaches, and

also the British littoral shore-plants, Cakile maritima, Crithmum

maritimum, Matricaria inodora, and Scirpus maritimus.

SECTION II. The buoyant tissue forms a layer inside the hard test of a

seed or inside the shell of the “stone” of a drupaceous fruit, and to

this cause the floating power is mainly or entirely due.

G. Mucuna gigantea (seed).

S. Hernandia peltata.

S. Excæcaria agallocha.

S. Cycas circinalis.

S. Pandanus odoratissimus.

G. Anona paludosa (seed) of tropical America.

_Note._—I have followed Schimper in respect to Pandanus, but it might be

by some placed in the first section of this group.

Here belongs Euphorbia paralias, a British littoral plant, the buoyant

seeds of which occur in the stranded seed-drift of English and

Mediterranean beaches.

In the following general discussion of the groups, reference will be

made only to the plants best illustrating the different varieties of

structure connected with buoyancy; whilst mention of the other plants

will in some cases be found in other parts of this volume, as shown in

the Index; and the matter is discussed at some length in not a few of

the species.

THE FIRST GROUP.

Of the first group, where the floating power is due to the unoccupied

space in the cavity of the seed or fruit, the Convolvulaceæ offer the

most typical examples. Here as a rule the crumpled embryo fills the

seed-cavity more or less incompletely; and it is on the relative size of

the unoccupied space that the sinking or floating of the seed depends.

In those plants where the seed sinks the seed-cavity may be almost

filled, as in Ipomœa tuberculata, or densely packed, as in Ipomœa

pentaphylla, and in species of Cuscuta. When the seed floats, as with

Ipomœa pes capræ, I. glaberrima, &c., the unoccupied space is relatively

large; and when, as with I. bona nox and I. turpethum, the behaviour of

the seeds is irregular, some floating, and others sinking, a

corresponding variation exists in the extent to which the seed-cavity is

filled. This applies also to the irregular behaviour of the seeds of

Ipomœa peltata and of Argyreia tiliæfolia. A singular instance is

afforded by the seeds of Ipomœa insularis, collected by me in Fiji and

Hawaii. Those from Fiji were incompletely filled, and consequently

buoyant. Those from Hawaii were more densely packed and sank.... The

three British species of Convolvulus illustrate the same principle,

namely, C. arvensis, with non-buoyant seeds; C. soldanella, with buoyant

seeds; and C. sepium, with seeds irregular in behaviour.

In the case of plants of the Convolvulaceæ, possessing buoyant seeds,

there is always evidence of marked shrinking of the seed-contents before

the final setting and hardening of the seed-coats. The embryo often

appears shrivelled and dried up, and is almost brittle, so that large

spaces are produced in the seed-cavity. If we partly divide such a seed

and place it in water, the embryo absorbs water rapidly, and within an

hour is soft, healthy-looking, and much swollen, the interspaces being

filled with a jelly-like mucilage. It is therefore evident that absolute

impermeability of the seed-coats is essential for the successful

transport by sea-currents of the floating seed; and we can only suppose

that the shrinking of the seed-contents takes place before the final

setting of the tests. That with the buoyant seeds the coats are quite

waterproof was illustrated in many of my experiments where, after a

period of flotation covering several months, and sometimes a year or

more, the seed-contents were still quite dry and shrunken. The limit of

buoyancy, as I have shown in Chapter IX., depends on an attempt at

germination on the part of the floating seed, which then absorbs water,

softens, swells, and sinks.

It is, therefore, not a matter of surprise that non-buoyant seeds of the

Convolvulaceæ do not gain floating power after prolonged drying of many

months. It is also to be expected that, as we find in Fiji, when a

characteristic shore-species with buoyant seeds like Ipomœa pes capræ

extends far inland, the seeds retain their floating powers.

Seed-buoyancy of this description is, on the face of it, purely

mechanical.

EXPLANATION OF THE DIAGRAMS ILLUSTRATING THE CAUSES OF SEED-BUOYANCY

1. _Entada scandens_ (natural size): (_a_), the shell; (_b_), the

kernel; (_c_), the intercotyledonary cavity. The shell consists of

three coats—an outer and an inner hard chitinous coat, and an

intermediate layer of brown cellular tissue containing little or no

air. The buoyancy is due entirely to the central cavity, neither the

seed-tests nor the seed contents possessing any floating power (see

page 181).

2. _Mucuna urens_, from Hawaii (natural size). The kernel (_b_) sinks,

and the shell has no floating power except where it possesses (under

the raphe) a layer of dark brown, air-bearing, spongy tissue (_a_).

This, however, is not sufficiently developed to endow the seed with

buoyancy, which is due to the intercotyledonary cavity (_c_). (see

page 111).

3., 4. _Mucuna gigantea_, from Fiji (natural size). The kernel (_b_)

sinks, and the seed owes its floating power entirely to the existence

in the shell (_a_) of a layer of brown, spongy, air-bearing tissue

which is mostly developed at the circumference and is almost wanting

at the flat sides of the seed (see page 115).

5., 6. _Dioclea_ (_violacea?_), from Fiji (natural size). Here the

kernel (_b_) is buoyant and endows the seed with floating power.

Though the shell (_a_) possesses a thick layer of reddish-brown

cellular tissue, this tissue contains but little air and aids the

floating power but slightly (see page 113).

7. _Strongylodon lucidum_, from Fiji (natural size). The floating power

is due entirely to the buoyant kernel (_b_). There is a very scanty

amount of loose brown tissue (_a_) under the raphe; but it has no

appreciable effect on the buoyancy (see page 113).

8., 9., 10. _Cæsalpinia bonducella_ and _C. bonduc_, from Fiji (natural

size). Neither the seed-tests (_a_) nor the kernel (_b_) have any

floating power in themselves, the buoyancy being connected with a

large internal cavity (_c_), which normally is intercotyledonary, as

in Fig. 8 (C. bonducella). With both plants, but more especially with

C. bonduc (Figs. 9 and 10), there may be a lateral cavity (_d_), or

the kernel may be loose in the shell (Fig. 10), but this does not

necessarily imply buoyancy (see page 194).

11., 12. _Arenaria peploides_ (enlarged: seeds 4 mm. in size). Here the

curved embryo (_a_) sinks, and the spongy air-bearing albumen (_b_)

gives buoyancy to the seed (see page 116).

13. _Euphorbia paralias_ (enlarged: seeds 3 mm. in size). The kernel

(_b_) sinks, and the seed owes its buoyancy to a layer of air-bearing

tissue (_a_) in the shell (see page 116).

14. _Morinda citrifolia_ (enlarged pyrene 7 mm. long). The floating

power is due to the bladder-like air cavity (_a_). The seed (_b_)

proper is enclosed in the woody tissue behind the bladder (see page

112).

15. _Cucurbita_ (seed enlarged), from the Valparaiso beach-drift (see

page 125). The kernel (_b_) has no buoyancy. The shell (_a_) is formed

of two layers of air-bearing tissue, the outer composed of prismatic

cells and the inner of a spongy vacuola-material.

[_To face page 111._

[Illustration: Diagrams illustrating some of the causes of

seed-buoyancy.]

Another type of the buoyant seeds of the first group is presented by

several species of Leguminosæ, as with Entada scandens, some species of

Mucuna, and Cæsalpinia bonducella. As with the Convolvulaceous seed, the

embryo sinks and the seed-shell has no buoyancy; but here the floating

power is due to the existence of a more or less symmetrical long central

cavity produced by the arching or bending outwards of the large

cotyledons which lie usually in close contact with the seed-shell. This

arching outward of the cotyledons depends on a shrinking process in the

setting or final stage of the maturation of the seed. The stages of the

process may be traced in the immature seeds, which are much larger and

in some cases twice the size of the mature seed. In this immature

condition the seed-coats are soft, and the flabby fleshy and thick

cotyledons fill up the seed-cavity. As the hardening and setting process

continues, the cotyledons diminish in size, become firmer, and gradually

bend outward, leaving a central cavity. This arching outwards is no

doubt in part the result of the contraction of the seed-tests during the

shrinking process. Considerable variation prevails in the results, and

where the cavity is very small the seed sinks. Further details relating

to this subject will be given in my treatment of some of the plants, and

especially under Cæsalpinia. But it may be here remarked with reference

to Hawaiian seeds of Mucuna urens D.C., that although they are strictly

referable to this group, they display beneath the hard test, on the side

beneath the raphe, a scanty layer of dark spongy air-bearing tissue

which is sufficiently buoyant to float up detached portions of the test,

but does not of itself give buoyancy to the seed. The significance of

this structure will be subsequently pointed out. The seed owes its

floating power to the large central cavity, but this layer of spongy

tissue adds to its buoyancy.

The section where the buoyancy of the fruit is connected with unoccupied

space in the fruit-cavity is extremely heterogeneous in its composition.

Every fruit has a method of its own, and the great variety of causes of

buoyancy of a mechanical character is here exemplified. For instance,

with Gyrocarpus jacquini and Cassytha filiformis the cause of buoyancy

is in the main the same as that described in the case of the

Convolvulaceæ. The origin of the floating power of the pods of Derris

uliginosa is two-fold. In the first place the seed or seeds but partly

fill the pod, and in the second place the seed is able to float of

itself by reason of its possessing, as in the seeds of Entada scandens,

a large central cavity produced by the arching out of the cotyledons

during the final stage of maturation. A double cause is also to be

assigned to the buoyancy of the fruits of Heritiera littoralis and of

Smythea pacifica, where, in addition to the unoccupied space produced by

the shrinking of the seed, the fruit-case itself floats, though nothing

but a mechanical explanation is to be given of the floating of empty

ligneous fruits.

One of the most suggestive types of buoyancy belonging to the first

group is presented by those cases, which are, however, not very

frequent, where the floating power is to be attributed to empty

seed-cavities produced by the abortion of the ovule or failure of the

development of the seed. A significant instance of this is afforded by

the fruits of Premna taitensis, a coast plant. The buoyant “stone” of

the drupe, which is often found afloat in the Rewa estuary in Fiji, is

4-locular, each cell containing normally one seed, but as a rule only

one cavity contains a mature seed, the three other cavities becoming

more or less empty through the failure of their seeds. It can be proved

that neither the seeds nor the substance of the “stone” are buoyant, and

that the “stone” owes its capacity of floating for months to the empty

cavities arising from the failure in development of three out of the

four seeds. In Fiji we see the rivers distributing these small fruits,

and we find the “stones” stranded on the beaches and floating in the

currents amongst the islands; and there can be no doubt that this is one

of the effective modes of dispersal of the species; yet, if there was

ever a case of accidental buoyancy concerned with dispersal by currents,

we have it here. Further details are given in Note 32.

It is probably also to the abortion of the ovule, or to the failure of

the seed, that the remarkable air-cavity (see Note 8) to which the

pyrenes of Morinda citrifolia owe their floating power, is to be

attributed. To this structure Professor Schimper (pp. 165, 183, 200)

attaches considerable importance as an example of special adaptation to

dispersal by currents through the influence of Natural Selection. He

suggests, however, that possibly its morphological significance may be

found in its being a peculiarly modified seed-chamber. The case of

Premna taitensis above cited indicates that the latter view is the most

probable. The subject awaits a careful microscopical study of the

seed-development of the genus Morinda since, as elsewhere remarked, the

non-buoyant pyrenes of inland species have not such an air-chamber. An

outline sketch of a pyrene of Morinda citrifolia is given in the

preceding plate. A good figure of it occurs in Schimper’s _Plant

Geography_, p. 28. A very suggestive instance of this nature is

described under Brackenridgea in Note 46 and in Chapter XIII.

THE SECOND GROUP.

Here are included those seeds and stone-fruits that possess buoyant

kernels. Professor Schimper points out that since this is a feature both

with inland as well as coast plants such a character cannot be viewed as

an adaptation to dispersal by currents. The plants concerned belong

mostly to the Leguminosæ, and we find here some of the most widely

spread of strand species, such as Canavalia obtusifolia and Sophora

tomentosa, as well as some of the giant climbers of the coast forests

belonging to the genera Dioclea and Strongylodon. The kernels when

divested of their coverings float buoyantly, but they soon absorb water

and sink usually in a day or two, a circumstance indicating that it is

to the impervious coverings that they indirectly owe their capacity to

keep the seed or fruit afloat. It is noteworthy that seeds of

Strongylodon lucidum from Fiji display beneath the raphe a trace of an

internal layer of loose cellular tissue which, however, has no

appreciable effect on the buoyancy; whilst with seeds of Dioclea

(violacea?) from the same locality there is a thick layer of loose

tissue which aids the floating power of the kernel but is not of itself

sufficiently aeriferous to buoy up the seed.

This leads one to refer to two other plants belonging to this group,

Calophyllum inophyllum (Guttiferæ) and Ximenia americana (Olacineæ),

where, though the floating power is mainly due to the buoyant kernel, it

is also aided by a layer of air-bearing tissue inside the hard shell of

the “stone” of the drupe. Professor Schimper places these fruits in the

third or adaptive group on account of the layer of buoyant tissue, but

it would be more correct to class them according to the predominant

cause of their buoyancy. It can be shown that with a non-buoyant kernel

the “stone” no longer floats. This double cause of the floating power

renders an explanation very difficult, since it would seem indefensible

to give conflicting interpretations of their nature. With Ximenia

americana there is another great difficulty. Its drupes are known to be

dispersed by fruit-pigeons (_Introd. Chall. Bot._ p. 46); and judging

from the rare occurrence of the “stones” in the drift there is good

reason to believe that bird agency in the Western Pacific is predominant

in the dispersal of the plant. It is by such test cases as this that we

must put to the proof the reality or non-reality of the influence of

adaptation on seed-buoyancy.

THE THIRD GROUP.

We have here those plants where the floating-power is entirely or mainly

due to an air-bearing tissue in the seed-tests or fruit-coats. Several

of the fruits are figured in Schimper’s _Indo-malayische Strand-flora_,

and one or two are figured in the English edition of his work on

_Plant-Geography_, p. 29.

In the first section, where the buoyant tissue occurs at the outside or

forms the periphery of the seed or fruit, are included several of the

most familiar of the littoral trees and shrubs of the Pacific islands,

such as Barringtonia speciosa, Cerbera Odollam, Guettarda speciosa,

Pemphis acidula, Scævola Kœnigii, Terminalia katappa, and several others

named in the synopsis. I cannot enter into detail here, but the reader

will find fuller particulars of each plant in most cases in Professor

Schimper’s work, and in some instances in my separate discussion of the

plants concerned. In nearly all cases we are concerned here with the

fruits, and only in a few cases with the seeds, as with Carapa and

Pemphis acidula.

This investigator observes that to this sub-group belong the fruits and

seeds usually described in systematic works as provided with corky or

suberous coverings; but he points out (p. 167) that the resemblance is

nearly always quite superficial, and is limited to colour and

consistence, suberous tissue occurring in only a few cases, as in the

fruit-coats of Clerodendron inerme. The buoyant tissues, he remarks, are

often more or less ligneous, and in those cases where there is no lignin

reaction they resist the action of sulphuric acid much more effectively

than pure cellulose; whilst in their physical characters, as well as in

their behaviour with reagents, they differ just as much from ordinary

cork. Thus, they are but little elastic and often easily crumble away;

whilst in large fruits, like those of Cerbera and Terminalia, they would

soon be stripped off entirely when subjected to the “wear-and-tear” of

transport by currents, if they were not traversed by numbers of stout,

tough fibres which hold the materials together. Where the buoyant

tissues are firmer, as with Clerodendron inerme and Cordia subcordata,

the fibrous framework is scanty or absent, whilst very small seeds or

fruits, like those of Tournefortia argentea and Pemphis acidula, where

the “wear-and-tear” would be comparatively slight, often possess no

protecting fibres in the buoyant tissues.

In one or two fruits, like those of Cerbera Odollam, these tissues

display large intercellular spaces; but in the majority of cases such

spaces are insignificant in size or absent altogether. Speaking

generally, however, there is, as Professor Schimper observes, great

similarity in the structure of the buoyant tissue in the coverings of

these fruits and seeds. The cell-walls are thin or only slightly

thickened, and detached air-bearing portions of the tissue will float

for many weeks. The great floating capacity of these fruits and seeds is

stated by this investigator to be entirely due to the tenacity with

which the air is retained in the covering tissues. It is, however,

noteworthy that in the case of Scævola Kœnigii the fruits are just as

well suited for dispersal by frugivorous birds as by the currents, a

significant circumstance discussed in the next chapter.

The second section contains those plants where the buoyant tissue occurs

inside the hard shell of the fruit or seed, such as is found, for

example, in Anona paludosa, Mucuna gigantea, Hernandia peltata, Cycas

circinalis, &c. Professor Schimper here includes Calophyllum inophyllum

and Ximenia americana; but I have before remarked that the buoyancy of

their fruits is mainly due to their buoyant kernels. This aeriferous

tissue forms a layer between the seed or nucleus and the hard outer

shell. It is described by the above-named authority as soft or friable

and dark brown. The cells contain air and may be closely arranged or

separated by small interspaces, their walls being neither woody nor

suberous.

_The structure of the buoyant seeds and seedvessels of the littoral

plants of the British flora._

The littoral plants with floating seeds or fruits form but a section of

the strand-plants of the British flora, scarcely a third, as is pointed

out in Chapter IV., of the total number. Though small in number they

exhibit great variety in structure; and notwithstanding that as far as

they have been examined they may all be referred to one or other of the

groups and sections of the classification adopted in the synopsis for

the plants of the Pacific islands, nearly every plant presents in the

structure of its seeds or seedvessels a type of buoyant structure

different from the others.

The first group is represented by the seeds of Convolvulus soldanella,

which owe their floating power to the incomplete filling of the

seed-cavity. The second group, where the buoyancy arises from the

buoyancy of the kernel or nucleus, is illustrated by the seeds of

Arenaria (Honckeneya) peploides, but in a fashion quite unique. The test

is thin but impervious, and has no buoyancy; the curved embryo also

sinks; and the floating power arises from the air contained in the loose

spongy albumen, around which the embryo is coiled (see figure). A more

normal component of the second group is represented in some Leguminous

seeds, perhaps of Lathyrus maritimus, that occur regularly amongst the

stranded seed-drift of the north coast of Devon. Here the kernel of the

seed is buoyant. The seeds of Euphorbia paralias are indebted for their

floating capacity to a layer of spongy tissue containing large

air-spaces placed between the kernel and the chitinous outer test,

neither of which possess any floating power (see figure). They thus

belong to the second section of the third group.

The fruits of Cakile maritima, Crithmum maritimum, Matricaria inodora,

and Scirpus maritimus, all belong to the first subdivision of the third

group where the air-bearing tissue exists in the peripheral coverings,

the seed or nucleus in all cases sinking. With Cakile maritima there is

a light spongy outer case of aeriferous tissue, which, however, soon

loses the epidermis, a circumstance that probably explains the limited

period of flotation of about a week. The walls of the mericarp of

Crithmum maritimum are composed of spongy cellular air-bearing tissue

with a persistent epidermis, and the floating powers of the fruits are

consequently great. The achenes of Matricaria inodora have beneath the

epidermis a layer of buoyant tissue, and their structure is similar to

that found with the buoyant achenes of littoral species of Wedelia,

plants of the same order of Compositæ that are found on the Pacific

islands. The cause of the floating power of the fruits of Scirpus

maritimus lies entirely, according to Kolpin Ravn, in the air-bearing

cells of the epidermis. The reader will find the results of my

experiments on the buoyancy of the seeds in Notes 16, 17, and 18.

_Summary of the Chapter._

(1) Following the main lines of Schimper’s classification of those of

the Indo-Malayan region which possesses for the most part the same

species, the buoyant seeds and fruits of the littoral plants of the

Pacific islands are classed in three groups: the _first_ where the

cavity of the seed or fruit is incompletely filled, the floating power

arising from the empty space; the _second_ where the buoyancy is derived

from the buoyant nucleus or kernel; and the _third_ where it arises from

air-bearing tissues in the coats of the seed or fruit.

(2) The first and second groups, in which the question of adaptation to

distribution by currents through the agency of Natural Selection is not

raised, since the same structural characters are found in seeds and

fruits of inland plants not dispersed by the currents, are termed the

mechanical or non-adaptive groups. The third is distinguished as the

adaptive group, because it is here that Schimper finds evidence in

favour of the Selection Theory.

(3) The first group is best represented by the Convolvulaceous and the

Leguminous types. In the former, which is well illustrated by Ipomœa pes

capræ, the seed-cavity is imperfectly filled by the crumpled embryo, the

result of the shrinking process during the final setting of the seed. In

the latter, which is exemplified by Entada scandens and Cæsalpinia

bonducella, the seed displays a large central cavity produced by the

arching outward of the cotyledons during the shrinking process

accompanying the last stage of the maturation of the seed. As an

instance of fruits belonging to the group, those of Heritiera littoralis

may be cited. An uncommon type is presented in the “stones” of the

drupes of Premna taitensis, and in the pyrenes of Morinda citrifolia,

where the buoyancy arises from empty seed-cavities resulting from the

failure of some of the seeds.

(4) The second group with buoyant kernels includes mostly widespread

Leguminous species, such as Canavalia obtusifolia and Sophora tomentosa.

(5) The third or “adaptive” group comprises many of the characteristic

littoral trees and shrubs of the Pacific islands, such as Barringtonia

speciosa, Guettarda speciosa, Terminalia katappa, Tournefortia argentea,

&c., that contain in their fruit-coverings a buoyant cork-like material

often bound together by fibres, but which proves on examination to

resemble cork only in appearance. In another type, illustrated by the

fruits of Cycas circinalis and the seeds of Anona paludosa, the buoyant

tissue forms a layer inside the shell of the seed or “stone.”

(6) Some fruits like those of Ximenia americana and Calophyllum

inophyllum illustrate both the so-called mechanical and adaptive

principles in their structure; whilst with the first-named species they

are as well adapted for dispersal by frugivorous birds and are known to

be a favourite food of fruit-pigeons. The same difficulty arises with

the fruits of some other characteristic littoral plants, as with Scævola

Kœnigii, the drupes of which are equally well fitted for dispersal by

birds and currents.

(7) The same general principles have been at work in determining the

structures concerned with the buoyancy of the fruits and seeds of

British littoral plants. Although the species are few in number they

exhibit in this respect great variety, eight species illustrating six or

seven types of buoyant structure.

CHAPTER XIII

ADAPTATION AND SEED-BUOYANCY

The question of the operation of Natural Selection.—Are there two

principles at work?—The presence of buoyant tissue in the seed-tests

and fruit-coats of inland plants, both wild and cultivated.—Useless

buoyancy.—The buoyancy of seeds and fruits is not concerned with

adaptation.—Summary.

WHEN we speak of a certain structure as an adaptation to dispersal by

currents through the agency of Natural Selection, it is necessary at the

outset to be quite clear as to what is implied. Professor Schimper, who

brought his great and varied knowledge of many other phases of

plant-life to bear on this subject, is careful to clear the ground of

preliminary erroneous conceptions in such a perspicuous and impartial

manner that we cannot do better than follow his guidance. There are, he

observes (p. 178), many mechanisms or contrivances in plants, which,

though they seem to have arisen with a fixed purpose, can in no wise be

regarded as having been developed for that end, since they were produced

in quite a different connection and have merely acquired a new or

supplementary function, of which they are the cause and not the effect.

This is very much the position that I have taken up for the whole

subject of the relation between plants and their dispersing agencies,

and it will be found discussed in Chapter XI. It involves, as I venture

to think, a dominant principle in the organic world, which it is one of

the objects of this work to emphasise, namely, that Nature in dispersing

plants habitually makes use of structures and capacities that were

originally developed in quite another connection. Behind this change of

function, this new purpose, lies the secret of the organic world. There

is for me no more pregnant fact in plant-life than the thistle-seed

blown before the wind, or the seed of our sea-convolvulus floating in

the sea. It proves to my mind that the evolutionary power in nature is

checked and hampered by conditions not of its own creation, and that two

opposing forces are ever at work, the one creating and the other

limiting the creative power, the actual mode of dispersal being but a

blind and accidental result of the struggle.

The question of the operation of Natural Selection is not raised, as

Professor Schimper indicates, until we consider whether the new function

has had any bettering influence on the structure or mechanism with which

it has come to be concerned. If such a modification is thus brought

about it might be legitimately claimed as a result of this agency, and

the term “adaptation” could be used. But if there is no evident change

produced, we should be compelled to assign very subordinate limits to

the capacity of Natural Selection; and in the instance of buoyant fruits

and seeds it would be restricted to determining a plant’s station by the

water-side and in increasing its area. It is only in the first case that

we could speak of them as adaptations in the meaning attached to this

term in the language of the Selection Theory. It would at first sight

seem easy to ascertain whether the characters of fruits and seeds, to

which the buoyancy is due, are adaptations in this sense of the word;

but in reality it is far from being so. We can, however, proceed with

unanimity up to a certain stage in the argument; but there agreement

ends.

It has been before established that in the Pacific islands, and indeed

in the tropics generally, the plants with buoyant seeds or seedvessels

are mainly stationed at the coast. It has also already been shown that

this littoral station is often associated with a special buoyant-tissue

in the coverings of the seed or fruit; and it will now be pointed out

that this tissue is, as a rule, absent or but scantily developed in the

case of inland species of the same genus. Of great importance, remarks

Professor Schimper (p. 179), in relation to the Selection Theory and the

development of adaptations, is the comparison of the fruits and seeds of

strand-plants with those of allied inland species; and he finds here

evidence in support of the Darwinian view. He takes the cases of the

genera Terminalia and Calophyllum, which contain both inland and

littoral species; and he shows that although the same buoyant-tissue

occurs in the fruit-coats of inland species, it is there much

diminished, and in consequence the floating powers are considerably

lessened or lost altogether (see Chapter II.). It is not pretended that

this tissue has had any connection in its origin with dispersal by

currents, but merely that its greater development in the shore species

is an adaptation to this mode of transport.

Further testimony is adduced by this investigator (p. 182) in supporting

his view in the fruits of the genera Barringtonia, Clerodendron, Cordia,

and Guettarda, where the buoyant tissues extensively developed in the

coast species are either non-existent or only represented by a trace in

the inland species of the same genus, a difference in structure

associated with the loss or great diminution of the floating capacity of

the fruits concerned. I have been able to establish other examples in

the cases of the genera Scævola and Tacca, which will be found referred

to in Chapter II.

Professor Schimper (p. 200) points to the circumstance that the

“adaptations” in these fruits all belong to the diagnostic marks of the

genera and the species, and contends that these plants abundantly prove

the erroneous nature of the contention that Natural Selection could have

played no part in the elimination of the strand-flora. My own contention

is that Natural Selection has played such a part, but that in doing so

it has merely availed itself of characters previously existing, without

originating, modifying, or improving them in any way. The foregoing

evidence might with equal fitness be employed to show, as pointed out in

Chapter II., that in the course of ages there has been a great sorting

process by which, excluding the mangroves, plants of the xerophilous

habit possessing buoyant seeds and fruits have been sorted out and

placed at the coast. Direct evidence does not lead us farther than to

the establishment of a littoral station for plants thus endowed. The

problem whether the characters of their fruits and seeds that are

concerned with buoyancy may be regarded as adaptive in the Darwinian

sense lies beyond the reach of direct testimony. We can, however,

approach it from the outside by several directions, and from some of

these we will now proceed to deal with it.

There is first the singular circumstance that in Fiji, when the littoral

plants with buoyant seeds or fruits leave the beach and extend far

inland, they, as a rule, retain their floating powers and, of course,

their buoyant structures. I found this to be true of Cassytha

filiformis, Cerbera Odollam, Ipomœa pes capræ, Morinda citrifolia,

Scævola Kœnigii, and one or two other plants mentioned in Note 44, where

this subject is discussed. My experiments on these plants indicated that

their fruits or seeds floated equally long, whether obtained from coast

or from inland plants. This, at first sight, appears to present a

serious objection to the adaptation theory; but it was not so regarded

by Professor Schimper, who in a letter to me, dated March 8th, 1900,

observed that he did not see “why littoral plants growing inland should

lose their adaptations to littoral life, especially if those adaptations

are not conflicting with the conditions of life beyond the littoral

zone, and if the competition does not require special adaptations.”

My view, however, is that any process of adaptation is unnecessary. All

these plants, it is contended, were originally inland plants that

acquired the buoyant qualities of their seeds and fruits in the inland

stations, and ultimately found a station at the coast through the

sorting process above referred to. In the case of plants like Ipomœa pes

capræ and Cassytha filiformis this would be conceded, since they belong

to the acknowledged non-adaptive groups discussed in the preceding

chapter. It is only to some of these plants, such as Scævola Kœnigii and

Cerbera Odollam, that the adaptation view of Professor Schimper is

applied; and the question arises whether we are justified in making such

a distinction, or, in other words, whether it is antecedently probable

that two independent principles have been at work in determining the

fitness of seeds and fruits for dispersal by the currents.

The plants for which the influence of adaptation through Natural

Selection is claimed belong, as stated in Chapter XII., almost entirely

to the third group. It is admitted that with the other two groups the

utmost that any sorting or selecting process would effect would be to

determine a station at the coast and to extend the area of distribution.

The numerical aspect of the question therefore acquires some importance;

and the reader’s attention is accordingly directed to the results

tabulated in Note 45, where it is shown (assuming for the time that

there is no difference of opinion about the adaptive significance of the

seeds and fruits concerned) that the plants of the third or adaptive

group make up only about half the total. It would therefore appear that

if the agencies of Natural Selection have been at work here either in

bettering or in developing buoyant structures, half of the shore-plants

with buoyant seeds or fruits have not come within their influence.

But the subject takes another aspect when we reflect that in some

buoyant fruits, as with Ximenia americana and Calophyllum inophyllum,

the two principles would seem to have been at work. Whilst from this

standpoint Natural Selection is regarded as having either developed or

increased in amount the layer of buoyant tissue in the fruit-coats, the

buoyant kernels are not viewed as adaptive in their origin. In the case

of Ximenia americana the dispersing agency of frugivorous birds adds

another factor, since, as before stated, its drupes are known to be

dispersed by fruit-pigeons. In the cases of Scævola Kœnigii and of Vitex

trifolia, two plants belonging to the adaptive group, Professor Schimper

(pp. 156, 188) admits also the dispersing agency of frugivorous birds,

and he claims it for Morinda citrifolia, in the pyrenes of which he also

detects a special adaptation to dispersal by currents. It may be added

that, as he also points out, fruits of the non-adaptive group of

littoral plants, such as Premna integrifolia (P. taitensis) and Cassytha

filiformis, would sometimes also attract birds. In fact, those of the

last-named have been found in the crops of pigeons (_Introd. Chall.

Bot._, p. 46).

Looking at all these littoral plants with fruits that are equally fitted

for dispersal by birds and by currents, we may now ask, Where does the

general principle of adaptation to dispersal lie? Whatever view we

adopt, we must apply the same view to all, whether it be a question of

dispersal by birds or by currents. We cannot choose between two sets of

principles determining the buoyancy of seeds and fruits any more than we

can regard a fleshy drupe and a buoyant seed as illustrating different

principles regulating the dispersal of plants. Nature works with

uniformity in these matters, and if the Natural Selection theory is held

to explain one case it ought to account for all. Yet nobody would go so

far as this; and this view of dispersal is on many grounds antecedently

improbable. These difficulties disappear if we assume that in all cases

the dispersing agencies have without modification made use of characters

and capacities that were developed, as we now see them, in quite other

connections and under quite other conditions.

It will now be necessary to look a little closer into the subject of the

buoyant tissue, to the existence of which in their coats about half of

the littoral plants concerned owe the floating power of their fruits or

seeds. In the first place, it is to be remarked that in the case of some

of the seeds of the plants of the non-adaptive groups it is also

represented to a small degree in the seed-coats, although, as with

Strongylodon lucidum and Mucuna urens, it is not present in sufficient

amount to float the seed. In the next place, it should be noted that

with some genera possessing, like Terminalia, both inland and coast

species it is to be found alike in the fruit-coverings of inland and of

littoral plants, though in a less degree in the case of the fruits of

inland species, the floating power of which is proportionately

diminished. There are, however, a few cases where this buoyant tissue is

developed in inland species which belong to genera or subgenera that

have no littoral species. This is what we would expect, if Natural

Selection has merely concerned itself with placing plants of xerophilous

habit possessing buoyant seeds or fruits at the coast. Under such

conditions we would now and then expect to find an inland plant

possessing buoyant fruits or seeds of this description that has never

been able to establish itself at the coast.

A good instance is afforded by Pritchardia Gaudichaudii, a fan palm

peculiar to Hawaii, the drupes of which float for several weeks and have

a covering of spongy buoyant tissue (see Chapter XXV.). The seeds of

Hibiscus Abelmoschus, a species distinguished subgenerically from the

littoral Hibiscus tiliaceus, offer another example. They float for

months, and owe their buoyancy to a layer of air-bearing tissue between

the kernel and the test, in this respect differing from the seeds of the

littoral species, where the floating power is due to unoccupied space in

the seed-cavity. The buoyancy of the seeds of Hibiscus Abelmoschus thus

offers another example of ineffectual floating power, since it is not a

littoral plant, is often cultivated, and has accompanied aboriginal man

over much of the tropical zone.

A singular instance of the dispersal by currents of an inland plant that

occurs both wild and cultivated in tropical America, the West Indies,

and on the West Coast of Africa, is afforded by Spondias lutea, Linn.,

which is referred to at the end of Chapter XXXII. Its “stones,” which

are provided with a cork-like covering much as we find with those of

Cordia subcordata and Guettarda speciosa, possess great buoyancy, and

are found in the river and beach drift of those regions with the seeds

in a sound condition.

A very remarkable case of ineffectual buoyancy is presented by the

seedvessels of Brackenridgea, which have been found floating in the

drift off the coast of New Guinea. They owe their floating power to

closed cavities which would seem to arise from the failure of one of the

seeds or from the abortion of an ovule. But, according to Beccari, their

fleshy coverings would aid their dispersal by frugivorous birds; and

since the species are all much localised and are rarely littoral in

their habit, it is very probable that birds have mainly effected the

dispersal of the genus (see Note 46). It has, however, been shown in the

previous chapter that Premna taitensis and Morinda citrifolia owe their

dispersal by currents to similar cavities in the seeds or “stones.”

Amongst the inland plants possessing seeds or fruits that are dispersed

by the currents without aiding the distribution of the species may be

recognised types of both the adaptive and non-adaptive groups. A

singular instance is afforded by the large seeds almost an inch long of

a huge pumpkin (Cucurbita) which, in sound condition, form one of the

commonest constituents of the beach drift on the coast of Chile from

Valparaiso northward to Iquique. The fruit is commonly eaten by the

lower classes. The seeds, which are very buoyant, contain a kernel that

does not float, the buoyancy being due to the water-tight coats which,

as shown in the plate in Chapter XII., possess well developed

air-bearing tissues. It may here be observed that Martins refers to the

germination of seeds of Cucurbita pepo after 45 and 93 days’ flotation

in sea-water.

One sometimes finds buoyant tissue developed in the seeds of

bottle-gourds, where it can serve no useful purpose of dispersal. Thus

small bottle-gourds, seemingly of the genus Cucurbita rather than of

Lagenaria, are to be commonly found afloat in the Guayaquil River and

stranded on the Ecuador beaches. They will float for many months, and

contain the seeds dried up into a small loose compacted mass in their

interior. These seeds, which contain a layer of spongy air-bearing

tissue in their coverings, will in several cases float for months. Some

that I had been keeping two months afloat in sea-water germinated

freely. It is shown in Note 47 that bottle-gourds containing sound seeds

are dispersed far and wide by the currents. In some species the seeds

are buoyant, and in others they sink in sea-water; but the gourds

themselves will float for probably a year or more, and the floating

capacity of the seeds when it exists is too insignificant to affect the

fruit’s buoyancy.

Other instances of the useless buoyancy of fruits of inland plants are

afforded by different species of Citrus. In the floating drift of the

Fijian rivers the fruits of the wild and indigenous Shaddock (C.

decumana) and of an inedible Orange, also wild and indigenous (C.

vulgaris?), are at certain times to be found, the latter often in

numbers. The first-named floats four to five weeks in sea-water, and the

last-named nearly two months, and both are to be observed floating out

at sea between the islands. The fruits of the Tahitian Orange, a variety

of C. aurantium, floated in sea-water between three and four weeks. The

seeds of these and other species of Citrus sank in from a few hours to a

day or two. The buoyancy of the fruit depends on the rind—the thicker

the rind the greater the floating power. This was not only shown in the

length of the period of flotation, but also in the buoyant behaviour of

the fruit. With the Tahitian Orange, where the rind is relatively thin,

the fruits floated heavily in sea-water and only protruded slightly

above the surface. With the Shaddock and with the other indigenous

species of Citrus, the fruits floated lightly and protruded half-way out

of the water.

There is nothing trivial in these examples of buoyant fruits. That they

have at times aided in the dispersal of the genus, with man’s assistance

in planting the seeds of the stranded fruits, I cannot doubt; but

unaided by man such buoyant capacities would be useless for purposes of

effective dispersal by currents. Between the two genera Terminalia and

Citrus there is this great distinction, that the former is more or less

halophilous, some of its species being at home on the sea-beaches,

whilst the latter, as Schimper would term it, is salt-shy, and includes

no halophytes or plants of the sea-shore amongst its species. The only

effect of buoyancy of the fruits on the distribution of the species of

Citrus would be to place them by the side of the river and the pond.

This has evidently been its result in the case of the Shaddock in Fiji,

where, as Seemann remarks, it often thickly lines the banks of the

rivers.

As also indicating that the buoyancy of the seed or fruit would never,

apart from the halophilous habit, endow an inland plant with a littoral

station, the examples of the Oak (Quercus robur) and of the Hazel

(Corylus avellana) may be taken. As shown in Note 48, these fruits

acquire floating power by drying, on account of the space formed by the

shrinking of the kernel. They occur commonly in beach drift, but rarely

in a sound condition; yet experiment has proved that they will sometimes

germinate after prolonged sea-water flotation. The fruits of other

species of Quercus are also transported in tropical regions by the

currents, but never, as far as I could learn, effectively. The Amentaceæ

as an order are “salt-shy,” and with only a few exceptions shun the

sea-beach.

In the great sorting-process, by which xerophytic plants with buoyant

seeds or fruits have been placed at the coast, and hygrophytic plants

with similar fruits or seeds have been stationed at the riverside or by

ponds and lakes, one might expect to find that other influences may have

at times been in conflict with the selecting operation here indicated.

To this cause may probably be attributed the cases of “useless buoyancy”

above referred to. Here we find in some inland plants fruits and seeds

with buoyant tissues in their coverings that in the case of littoral

plants would have been regarded as the result of adaptation to dispersal

by currents. Such cases go to emphasize the conclusion already indicated

that these tissues could not have been developed through the agency of

Natural Selection. But the great objection against the application of

the Darwinian view to the general subject of the buoyancy of the seeds

and fruits of littoral plants lies in the circumstance that quite half

of the plants concerned are admitted to be outside the scope of the

theory, and that for these another explanation has to be found. I think

we may fairly claim that in a matter which finally resolves itself into

a question of buoyancy one explanation should cover all. We have thus to

decide whether to regard as adaptations to dispersal by currents the

structures of the buoyant seeds and fruits of littoral plants; or

whether to hold the view that as far as dispersal by currents is

concerned such structures are purely accidental, and that Nature has

never directly concerned herself in the matter at all. The first

explanation lies under the disadvantage above alluded to, and it remains

to be learned whether the second view could be made to cover all cases

of dispersal by currents. Further investigation on many points is yet

required; but, apart from the evidence against Natural Selection as the

principal agency that has been produced in this chapter, a powerful

argument in favour of the view that the buoyancy of seeds and fruits is

not concerned with adaptation is, that as a rule the floating capacity

of the seed or fruit has no direct relation with the density of

sea-water. Generally speaking, as shown in Chapter X., these seeds and

fruits are much more buoyant than they need to be, that is to say, if

they owe their floating power to adaptation to dispersal by currents.

This is quite in accordance with the argument developed in Chapter XI.

with regard to the general question of plant-distribution, that

dispersing agencies make use of characters and capacities of seeds and

fruits that were never intended for them.

_Summary of the Chapter._

(1) There are many mechanisms or contrivances in plants that now serve a

purpose for which they were not originally developed.

(2) Of this nature, it is contended, is the relation between fruits and

seeds and the agencies of dispersal.

(3) If, however, the structure or mechanism is made more effective by

the new function, such a modification may be regarded as an “adaptation”

in the language of the theory of Natural Selection.

(4) It is held by Professor Schimper that the structures connected with

the buoyancy of the fruits or seeds of several tropical littoral plants

are, in the above sense, adaptations; and he points to several genera

where the buoyant tissues in the coverings of the fruits or seeds of the

coast species are scantily represented or absent in the inland species

of the same genus, a difference corresponding with the loss or

diminution of the floating powers.

(5) This contrast in structure and in floating capacity between the

fruits or seeds of inland and coast species of the same genus is beyond

dispute, and the author adduces fresh data in support of it.

(6) But he contends that it is not proved that the relatively great

development of buoyant tissues in the case of littoral plants is the

effect of adaptation; and that if the selecting process had been

confined to sorting out the xerophilous plants with buoyant seeds or

fruits and to placing them at the coast, the same contrast would have

been produced.

(7) In support of this contention he points out that when such littoral

plants extend inland the floating capacity and the buoyant tissues are

as a rule retained; and that in those exceptional cases where inland

plants possess buoyant fruits or seeds these tissues are sometimes well

developed under conditions in which they could never aid the plant’s

dispersal.

(8) But the most serious objection against the adaptation view is that

admittedly only about half of the shore-plants with buoyant fruits or

seeds come within its scope. Therefore a second explanation has to be

framed for the other plants concerned.

(9) As showing the difficulties raised by regarding some of the

structures connected with buoyancy as “adaptive” and others as

“accidental,” it is pointed out that some fruits possess the two kinds

of structure. It is also shown that in several cases fruits endowed with

buoyant tissues are just as well adapted for dispersal by frugivorous

birds; and the instance of Ximenia americana is cited where a drupaceous

fruit, known to be dispersed by fruit-pigeons, possesses also in its

“stone” both the “adaptive” and “non-adaptive” types of “buoyant

structures.”

(10) It is urged that whatever is the relation between the buoyancy of

the seeds and fruits of shore-plants and dispersal by currents, there

has been a uniform principle affecting all.

(11) The weight of evidence is regarded as adverse to the Natural

Selection theory, an inference which is consistent with the conclusion

arrived at in Chapter X. that there is no direct relation between the

density of sea-water and the buoyancy of seeds and fruits, the floating

capacities being as a rule far greater than the adaptation view would

explain. Nature, it is held, has never made any provision for dispersal

by currents, the buoyancy of seeds and fruits being, as concerns the

currents, a purely accidental quality.

CHAPTER XIV

THE RELATION BETWEEN LITTORAL AND INLAND PLANTS

Professor Schimper’s views.—Great antiquity of the

mangrove-formation.—Problem mainly concerned with the

derivation of inland from littoral plants.—Grouping of

the genera possessing both coast and inland

species.—Scævola.—Morinda.—Calophyllum.—Colubrina.—Tacca.—Vigna.—Premna.

IN discussing the relation between the littoral and inland floras in the

Pacific it will be at first necessary to pick up some of the threads of

the various lines of investigation dealt with in the previous portion of

this work. Apart from considerations connected with the genetic history

of the plants concerned, when we come to inquire into the sources of any

individual strand-flora, whether in the temperate or in the tropical

regions, we arrive at the rough and ready inference that it is composed

of “what the sea sends and the land lends.” But it has been already

shown that the relative proportion of the current-borne and in

consequence widely dispersed plants in a strand-flora varies greatly in

different regions. Thus in the Pacific islands, as typified by those of

Fiji, about 90 per cent. have buoyant seeds or seedvessels originally

brought from distant localities; and in the tropics, as a rule, the

average would probably be never under 75 per cent. On the other hand, in

a temperate region the plants derived from inland would be most

predominant, making up probably some three-fourths of the whole, whilst

the proportion of current-dispersed plants hailing from distant places

would be relatively few.

It is on this account that there is such uniformity in the general

composition of the strand-flora over a large part of the tropics, since

current-dispersed plants are widely spread. But in the temperate regions

we find a great contrast in this respect. There are, it is true, a few

current-borne plants that one meets everywhere. For instance,

Convolvulus soldanella is to be gathered on English beaches and on those

of New Zealand and of the coast of Chile. But these littoral plants with

buoyant fruits hardly give a feature to the strand-flora. A multitude of

intruders, either characteristic of the inland flora of the region or

confined only to the seaboard of that part of the world, also make their

home on the beach and frequently endow a beach-flora with its leading

features. The possible associations of plants on a beach in a temperate

region are thus very great; and I have already discussed this in part in

Chapter IV. as concerning the British shore-flora. One has only to look

at a work like that of Dr. Willkomm on the vegetation of the strand and

steppe-regions of the Iberian peninsula to realise how the few littoral

plants familiar to the English eye cut but a sorry figure amongst the

numbers of strange intruders from the arid regions inland. So again, as

I found on the Chilian beaches, Convolvulus soldanella finds odd

associates amongst the species of Nolana and Franseria that are peculiar

to the coasts of that part of the globe (see Chapter XXXII.); and

different grotesque American forms of the Cactaceæ with a

Mesembryanthemum and a host of strange-looking plants descend from the

arid slopes of the hills behind to keep company with the far-travelled

English beach-plant (see Note 49). Or again, a glance at the pages of

Professor Schimper’s great work on _Plant-Geography_ will bring the same

fact home in a still more varied fashion.

Yet on tropical coasts the intruding inland element is also

distinguishable, though it may influence only to a small degree the

general character of the strand-flora. Dividing it, as we have described

in Chapter V., into the plants of the sandy beach and of the

mangrove-swamp, we find in the mangroves the most stable element and in

the beach-plants those most liable to change. Professor Schimper

observes that whilst the physiognomy of the beach-flora varies to some

extent with the alterations in the inland flora, the mangrove-formation

makes but a slow response to such changes. As he points out in his work

on the Indo-Malayan Strand-Flora (p. 199), seeds and seedvessels are

being continually brought down to the sea-coast through the agencies of

rivers, winds, and birds; and in this manner, in the course of ages, the

beach-flora is recruited from the inland plants. But for the mangroves

such additions to their numbers are rarely possible. Whilst the same

genera are often shared by both the beach and inland floras, we have in

the mangrove-formation families, sub-families, and genera almost

peculiar to itself, and including plants, like those of the Rhizophoreæ,

that in their characters betray but little kinship with others and give

but little indication of their descent. The mangroves have remained

through the ages as something apart from other coast-plants, isolated

both in their history and in their characters, and especially

distinguished by their “adaptations” to their surroundings.

Such is the line of argument followed by this eminent German botanist in

his account of the development of a tropical strand-flora. In various

parts of this work I have ventured to suggest that the mangroves may be

the remnant of an ancient flora widely distributed over the lower levels

and coastal regions of the globe in an age when vivipary (meaning,

thereby, germination on the plant) was the rule rather than the

exception. At such a period, as I imagine, the climatic conditions of

the earth were much more uniform than they are at present, at least in

the lower levels; and a warm atmosphere, charged with aqueous vapour and

heavy with mist and cloud, enveloped a large portion of the globe. The

mangroves, it may be remarked, are by no means universally distributed

on tropical coasts in our own time. (Professor Schimper describes their

distribution in his _Indo-malayische Strand-Flora_, pp. 85, 86, and in

the English edition of his _Plant-Geography_, p. 409.) They are not

found on rainless coasts even when under the Line, except where there

happen to be large estuaries; but where a rank and luxuriant inland

flora betokens a high degree of humidity, there they thrive. This is

well illustrated on the rainless shores of tropical Peru, a locality

described in Chapter XXXII. of this work.

Yet if, as it is here contended, the mangroves form a remnant of a once

widely spread viviparous flora, it might be expected that the

beach-plants of that age would have been also viviparous, and that with

their present descendants, as well as with some of the inland plants

allied to them, we ought to find in the anomalous structure of the seed

some indication of the lost viviparous habit. This appears to be the

case, as described in Note 50, with the Barringtoniæ, a tribe that has

supplied some of the most characteristic beach-trees, and also with some

genera of the Guttiferæ. Perhaps, indeed, when the seeds of several

other littoral beach-trees come to be examined, for instance, Guettarda,

analogous structures may be found.

Although the beach-flora of the tropics is less stable in its

composition than the mangrove-formation, it is not to be assumed that in

the Pacific region or in the tropics generally it is at all modern in

its character. Though in the main, no doubt, more recent than the

mangroves, since it is likely that in early geological periods the swamp

rather than the sandy beach formed the predominant feature of the

sea-border throughout the tropics, yet it bears in several respects the

impress of a high antiquity. There are few beach plants in the tropical

Pacific that are not found over the tropics of a large portion of the

globe, a circumstance that would in itself warrant our assigning a great

age to the beach-flora; and it is highly probable that some at least of

the beach plants of the Pacific that occur on the east and west coasts

of tropical America are, for reasons given in Chapter XXXII., older than

the barrier now interposed by Central America between the Atlantic and

Pacific oceans. There are, it is true, a few species, like Acacia

laurifolia and Drymispermum Burnettianum, which, on account of their

restriction to the beaches of the Western Pacific and their lack of

capacity for dispersal by currents, may be regarded as local

productions; but for the great majority, ranging as they do over much of

the tropics, it is not possible to determine when and where they assumed

their littoral habits. That except in a few instances their home in some

bygone age lay outside the Pacific can scarcely be doubted.

It is therefore to be expected that in a discussion of the relation

between the strand and inland floras in the Pacific islands the problem

will be mainly concerned with the possible derivation of inland from

littoral plants. In such a discussion the relation between the beach and

inland species of the same genus becomes a subject of great interest. It

is a subject that had a peculiar fascination for Professor Schimper, who

refers to it more than once in his pages; and though never able to take

it up, he viewed it as a very promising field of inquiry. The question

has been frequently alluded to in this work; and it is especially dealt

with in one connection in Chapter II. It is there shown that whilst, as

a general rule, the seeds or seedvessels of the coast species possess

great floating power, those of the inland species of the same genus have

little or none, and that both may have independent modes of dispersal,

the first by currents, and the last through frugivorous birds.

A close connection between the beach and inland floras is apparently

displayed in the circumstance that quite a third of the genera of the

Pacific insular floras containing littoral species (some 70 in all,

excluding the mangroves) possess in this region also inland species. But

the further examination of this interesting group of genera, which are

enumerated in the list below, goes to show that the connection between

the inland and coast species of a genus is by no means always so close,

or of such a character, as one might have expected. It will not be

possible, however, to do much more than indicate in this chapter the

results of this inquiry; but the details will usually be found either in

the separate discussion of the genus or in other parts of this work. For

convenience of treatment these genera may be grouped in the following

sections.

_Grouping of the Plant-Genera of the Islands of the Tropical Pacific

that possess both Littoral and Inland Species._

Section I. Where the littoral and inland species are most probably of

independent origin, both possessing their own means of dispersal;

Calophyllum, Hibiscus, Colubrina, Morinda, Scævola, Cordia, Ipomœa,

Vitex, Tacca, Casuarina.

Section II. Where the littoral species have probably given rise to

inland species, and both still exist in the group of islands: Vigna,

Premna.

Section III. Where inland species have been probably developed from

littoral species no longer existing in the group: Canavalia, Erythrina,

Sophora, Ochrosia.

Section IV. Where the littoral and inland species are evidently of

independent origin, and there is no means of accounting for the

existence of the inland species by agencies of dispersal at present in

operation: Barringtonia, Pandanus.

Section V. Where in the same genus some inland species are derivatives

of the coast species and others are of independent origin: Guettarda.

Section VI. Where the coast species, having little or no capacity for

dispersal by currents, are regarded as derived from the inland species

in one group of islands and as afterwards distributed to those in the

vicinity: Eugenia, Drymispermum, Acacia.

SECTION I

This group, which includes those genera where the coast and inland

species are regarded as of independent origin, both possessing their own

means of dispersal, contains about half of the total number of genera

here concerned. We will first deal with the genera Calophyllum, Morinda,

and Scævola, where the littoral species have buoyant fruits or seeds

that are dispersed by currents, whilst the inland species have more or

less non-buoyant fleshy fruits that could only be dispersed by

frugivorous birds. Here the inland and coast species could have arrived

independently at the island, and we are not called upon either on this

ground or by reason of affinity of characters to connect the one with

the other.

The genus Scævola is very typical of its kind and has been already in

part discussed in Chapter II. The wide-ranging shore-species, S.

Kœnigii, that is distributed over the Pacific may sometimes, as in

Hawaii, be accompanied by numerous inland species, all endemic, seven of

them being enumerated by Hillebrand; or, as in Fiji and Tonga, there may

be associated with it a solitary inland species, S. floribunda (see Note

51); or, as in Tahiti, it may exist by itself. On the other hand, as in

the Kermadec Islands, a single inland peculiar species may alone

represent the genus. The inland species have fleshy drupes which, as far

as examined, have no floating power and possess no buoyant tissues in

their coverings; and their independent dispersal by birds cannot be

doubted. The endemic character of most of the inland species of the

Pacific islands is most probably due to the suspension of the

transporting agency of frugivorous birds, just as the wide range of the

solitary littoral species may be attributed to the uninterrupted agency

of the currents. There is nothing in the description of the endemic

species given in Hillebrand’s _Hawaiian Flora_ to indicate any especial

genetic connection between the inland species and the beach plant, S.

Kœnigii; and the occurrence of a solitary inland peculiar species in the

Kermadec Islands clearly proves an origin independent of any littoral

plant.

Morinda is another critical genus in this discussion. Besides the

widespread littoral species (M. citrifolia) that is distributed by the

currents and is also dispersed by man, there are in the Pacific islands

a number of inland species, mostly climbers and denizens of the forests.

In the _Index Kewensis_ six are accredited to Fiji and five to New

Caledonia. Hillebrand gives a peculiar Hawaiian species, and there is a

widespread species (M. Forsteri) that ranges over the South Pacific from

New Caledonia to the Marquesas and the Paumotu Islands. Since, as

indicated in Chapter II. and in Note 8, the pyrenes of the fruits of the

inland species are not dispersed by the currents and could readily be

transported by frugivorous birds, we are not called upon to connect them

in their origin with M. citrifolia, the wide-ranging species of tropical

beaches.

The fact of the dispersal of certain inland species of the genus over

large areas of the tropics, such as in the case of Morinda umbellata

through tropical Asia and Malaya, and M. Forsteri in the Pacific, is

indeed sufficient proof that these inland plants are independent of any

littoral species in the Pacific and possess their own means of

distribution. Though the genus, comprising at least forty species, is

mainly confined to the Old World, there are a few species in America;

but M. citrifolia, the familiar beach species of the Old World and the

Pacific, is not indigenous there, and, as far as I can gather, all the

American species belong inland. Facts of distribution of this nature

negative the possibility that the Pacific islands have received their

inland species of Morinda through the intervention of the far-ranging

littoral plant.

As respecting Calophyllum, which is represented all over the tropical

South Pacific by the wide-ranging C. inophyllum and by a tree of the

inland forests found also in Malaya and in Ceylon (C. spectabile), there

are, apart from questions of affinity, grave objections against the

derivation of the same inland species from the coast species all over

this area. The fruits of the two inland species of Fiji, C. spectabile

and C. burmanni, have sappy outer coverings and are quite suited for

dispersal by fruit-pigeons. As observed in Chapter II. and Note 9, they

have limited floating capacities and their dispersal by birds is

necessary to explain their distribution. Since the timber is greatly

valued by the Polynesians, it is not unlikely, however, that those

islanders have assisted in the distribution of the inland species. It is

not possible to do more than touch on this subject here; but it may be

inferred that the history of Calophyllum in the Pacific has not been one

that would warrant our regarding the inland trees as derivatives of a

coast species.

There are other genera of this section where, for reasons of a different

character, there is no cause for assuming that the inland species are

derived from the coast species, or _vice versâ_. Thus, in Fiji,

Casuarina equisetifolia, a widely distributed species of the Old World,

occurs at the coast and in the scantily wooded plains behind; while C.

nodiflora, a New Caledonian species, finds its home in the lower

forests. There are many endemic species in Australia and New Caledonia;

and we are not called on to connect together these two species in Fiji.

In the same way we are not under any obligation in the case of the

numerous inland species of Ipomœa of the Pacific islands to connect them

with the coast species. They are all widely ranging species, and their

seeds have been carried to the islands, each in its own fashion. So

again with the inland species of Hibiscus found in the Polynesian

islands and often cultivated, we cannot either from the point of view of

dispersal or of affinity connect them with the far-ranging littoral

species, H. tiliaceus, which belongs to a section of the genus distinct

from those sections to which the inland species belong.

In a similar way there is no ground for supposing that Cordia aspera, an

inland species confined to Fiji, Tonga, and Samoa, is derived from C.

subcordata, the widely distributed littoral species of the Pacific and

of the Old World, since they belong to different sections of the genus.

But, apart from any question of affinity, the drupes of inland species

of Cordia are known to be well suited for dispersal by frugivorous

birds, though, unlike the littoral species above named, not adapted for

transportal by the currents. The genus Vitex, which is represented by a

wide-ranging littoral species in the Pacific (V. trifolia), appears to

be associated with inland species only in Fiji, where one or two,

seemingly endemic, occur. But there is nothing in Dr. Seemann’s

description of V. vitiensis, one of these species, that at all suggests

its derivation from the strand species, a very variable plant that often

extends far inland into the plains, adopting a different habit of growth

in those localities. It is known that Vitex fruits can be dispersed both

by birds and by currents. This genus is more fully discussed in a later

chapter.

Of the genus Colubrina there seem to be only two Pacific species

known—one the widely distributed shore-plant, C. asiatica, a straggling

shrub with alternate leaves found in all the Pacific groups and on the

beaches of much of the tropics of the Old World; the other a tree, C.

oppositifolia, with opposite leaves, that is peculiar to the Hawaiian

islands, where it frequents the open-wooded and scrubby inland

districts. The seeds of the shore-plant float unharmed for many months,

whilst the fruits of the inland plant, which differ in some important

respects (see Note 52), would float only for a week or two. The strand

species is also quite at home inland in many parts of the world; and

there is nothing from the standpoint of affinity to indicate that in

Hawaii it has given birth to an inland species so divergent in habit and

in character. There is of course the difficulty of explaining how a

plant like C. oppositifolia, with such a dry, unattractive fruit, could

be indebted to birds for its original introduction into the group; but

the same difficulty arises with a host of Hawaiian plants. It is,

however, evident from its distribution over the islands of this

archipelago that it possesses or has possessed some means of

inter-island dispersal, and since it is not of much service to the

aborigines we must look therefore to the bird.

In the instance of the genus Tacca there is in Fiji an inland species,

T. maculata, associated with a wide-ranging beach species, T.

pinnatifida, which also grows inland. The first-named is recorded from

the north coast of Australia and from Samoa, and though, unlike the

beach plant, its seeds are unfitted for dispersal by currents (see

Chapter II.), they might be distributed by birds. Dr. Reinecke describes

another inland species from Samoa, T. samoensis. The beach plant, T.

pinnatifida, grows so typically (sometimes side by side with T.

maculata) in the inland plains of Fiji that one would not be justified,

apart from questions of affinity, in regarding it as the parent form of

inland species in the Pacific islands.

For food and other purposes Tacca pinnatifida is or was much valued by

the Pacific islanders, and it grows so abundantly that cultivation is

rarely practised. That the Polynesians have aided the currents in the

distribution of the plant there can be no doubt, and this is

particularly indicated by its occurrence in Hawaii. The genus contains

ten or a dozen species, of which at least three are peculiar to America;

but T. pinnatifida, the characteristic shore-plant of the Old World, and

according to Schimper the only one that can be so designated, is not

found in America, where, as far as I can gather, there is no

widely-spread beach species dispersed by the currents from which the

peculiar species could have been derived. In the case of the Pacific

species, however, it should be noted that I am not endeavouring to prove

the improbability of the inland species having been derived from the

coast species in other regions, as in Australia, but that my point is to

show there is no reason to suppose that this has taken place in the

Pacific. There is no difficulty in attributing the dispersal of inland

species to birds; and we are therefore not called on to connect them

with the beach plants.

SECTION II

This division includes those genera where the littoral species has

apparently given rise to one or more inland species and both still exist

in the same group of islands. Two genera alone, Vigna and Premna, come

into this category. The first-named seems to present a good case for the

derivation of an inland from a coast species in Hawaii. Besides Vigna

lutea, the beach species, which is found not only all over the Pacific

islands but on the tropical beaches of the Old World, there are in

Hawaii two endemic species (V. sandwicensis and V. oahuensis) that occur

in the mountains, usually at elevations of from 1,500 to 5,000 feet; but

I do not find any more inland species recorded from the other Polynesian

archipelagoes. It may at first be noted that Vigna lutea, which in some

parts of the world strays inland, displays considerable variety in its

littoral station in the Pacific. Thus, in Hawaii, I found it sometimes

on the sandy beach, sometimes on a rocky shore, and sometimes on the

edge of old lava-cliffs overlooking the sea. In Fiji, though usually a

trailer on the beach, it may become a climber hanging from the trees

bordering the creeks in the mangrove-swamps. Though Hillebrand makes no

mention of forms intermediate between coast and inland species in

Hawaii, I found in one locality at the coast some specimens of Vigna

lutea displaying the twisted pods and two callosities on the standard

that are characteristic of V. sandwicensis, one of the inland species.

The seeds of Vigna lutea float in sea-water unharmed for months, and

they are to be found in the stranded drift of the Hawaiian and Fijian

beaches, and floating in the drift of the Fijian rivers. I was unable to

obtain the mature seeds of the inland species, and it has therefore yet

to be determined whether they follow the rule in the loss of buoyancy.

It may be added that a plant of Vigna lutea raised in Hawaii from seed

displayed some small tubers of the size of a pea on its roots.

The case for Premna is stated in Note 32. In this genus, as with Vigna,

the final test of experiment is needed; but the data at my disposal

point to the probability that an inland species has here been derived

from a littoral plant.

The summary of this chapter is given at the end of Chapter XVI.

CHAPTER XV

THE RELATION BETWEEN LITTORAL AND INLAND PLANTS

(_continued_)

Inland species of a genus developed from littoral species originally

brought by the currents but no longer existing in the

group.—Illustrated by the Leguminous genera, Erythrina, Canavalia,

Mezoneuron, and Sophora, and by the Apocynaceous genus, Ochrosia.—The

Hawaiian difficulty.

SECTION III

HERE we have three genera of the Leguminosæ, namely, Erythrina,

Canavalia, and Sophora, and one Apocynaceous genus, Ochrosia, in which

it is considered that inland species have been probably developed from

littoral species no longer found in the group. In this case the shore

species, possessing buoyant seeds or fruits that are known to be

dispersed by the currents, is absent from the particular group in which

the inland species occurs; and since the last-named displays no capacity

for distribution by currents, or seemingly by birds, we are driven to

infer that it was originally derived from a coast species, brought by

the currents, that has since disappeared.

Hawaii is the only region concerned here; and these four genera may be

said to well illustrate the particular “Hawaiian difficulty.” If this

explanation of the origin of the inland species is legitimate, then it

offers us a mode of explaining still more perplexing cases in the

Hawaiian flora, such as those relating to the endemic species of

Mezoneuron (Leguminosæ) and to Hillebrand’s Vallesia (Apocynaceæ), where

there is apparently no littoral species known from any region.

Dealing with the three Leguminous genera, it is at first to be remarked

that the great floating powers of the seeds of the littoral species are

in all three cases to be attributed to the buoyant kernel; whilst on

account of the non-buoyancy of the kernel the seeds of all the inland

species possess no floating power. Some very interesting points are

raised in each of the three genera, and I will first deal with the genus

Erythrina.

ERYTHRINA.

If we look over the Pacific islands in search of a critical locality for

the investigation of the genetic relation between the littoral and coast

species of Erythrina, we discover it, as far as I can gather, only in

one group. In Fiji, Tonga, and Samoa we find only the littoral species;

in Hawaii there is only an inland species; whilst in Tahiti occur both

the littoral and the inland species—E. indica, the wide-ranging

shore-tree of the South Pacific, and E. monosperma, the inland tree of

Hawaii—the last found nowhere else in Polynesia, and confined to the

Pacific. In Tahiti there are no other species, and it is between these

two species that the connection, if it exists, is to be sought. (Further

details relating to the genus are given in Note 53. In this place only

the facts bearing on the argument will be discussed.)

The buoyant seeds of Erythrina indica are well known to be dispersed by

the currents; whilst those of E. monosperma, as obtained from Hawaii,

have no floating power and sink at once, or in a day or so, even after

drying for two years. In Tahiti the first-named species is a

characteristic plant of the beach, whilst the last grows there in the

valleys and on the mountains up to elevations of 700 to 800 metres. We

have now to inquire whether there is any decided affinity between the

two species, and whether the divergent characters of the inland species

can be connected with its station. With regard to the first query we may

quote in reply the observation of Drake del Castillo, that as concerning

the foliage and the inflorescence E. monosperma is very nearly related

to E. indica, differing only from it in the more hairy calyx, in the

more permanently tomentose and much shorter pod, and in the paucity of

seeds (one or two in number).

We will now see whether it is possible to connect these differences in

character with differences of station. Neither Nadeaud nor Drake del

Castillo give precise descriptions of the station of Erythrina

monosperma in Tahiti; but Nadeaud and Lepine remark that it grows on

precipices as well as in the valleys on the north or dry side of the

island; and we may infer that it affects exposed dry rocky stations. In

Hawaii, according to Hillebrand, it is found on the dry rocky hills and

plains of all the islands up to 1,000 feet. I was particularly

interested in this tree whilst in the group, and found it in the large

islands of Maui and Hawaii thriving in rocky arid districts of little

rainfall, accompanied by Cactus opuntia, Ricinus communis, and

Cæsalpinia bonducella. It is often to be observed on scantily vegetated

lava-flows, a solitary tree growing here and there out of a crack in the

old lava, or it may dot the rocky slopes of some barren declivity. I

found it in the dry gulches behind Lahaina at elevations of 800 to 1,200

feet above the sea, growing amongst huge blocks of stone in clumps of

ten or twelve trees. When one contrasts the inland station of E.

monosperma with that of E. indica on the beach where the atmosphere is

more humid and the conditions more suited for plant-growth, it appears

probable that the differences between these two species may be largely

connected with station, especially as regards hairiness and the

diminished size of the pods.

Assuming, therefore, that Erythrina monosperma is but the inland form of

E. indica and that the differences between the two species are mainly an

affair of station, we have next to account for the occurrence of the

inland species in Hawaii without the littoral species. The agency of

currents in explanation of the existence of E. monosperma in Hawaii is

at once excluded, since the pods dehisce on the tree, and the seeds, as

already remarked, have no floating power. Nor does it seem likely that

beans half an inch (13 mm.) long could be transported unharmed in a

bird’s stomach over the two thousand miles of sea that intervene between

Tahiti and Hawaii. Yet one cannot doubt that the pyrenes and “stones” of

genera like Coprosma, Nertera, Cyathodes, and Osteomeles have been

carried by frugivorous birds to Hawaii. But a bean is somewhat different

from the crustaceous pyrene of Coprosma or the hard “stone” of

Cyathodes; and although, as indicated by the occurrence of an endemic

species of Erythrina in Fernando Noronha, birds may carry large beans

unharmed over a couple of hundred miles of sea, one hesitates to

conclude that they could effect this when the tract of ocean to be

traversed is ten times as great. There are again reasons for believing

that the seeds of Erythrina monosperma are particularly ill-suited for

dispersal by birds, since, notwithstanding their hardness, they soon

absorb water through the micropylar opening; and they germinated so

readily in my experiments that the digestive juices in a bird’s stomach

would probably soon find access and destroy the kernel. It is, however,

known from the observations of the Messrs. Layard in New Caledonia that

a small crow and different species of parrots feed on the seeds of

Erythrina, and they may aid in the local dispersal (_Ibis_, vol. 6,

1882).

To admit man’s agency in carrying to Hawaii the seeds of a tree which is

only useful in supplying him with light wood for his outriggers and his

fishing-net floats would compel us to place in the same category a great

number of plants in some way useful to him which are recognised as

indigenous. The Polynesian ransacks the vegetable world for his wants,

and carries with him in his migrations only his food-plants and the

seeds of his sacred trees.

There remains then the possibility that the parent species, Erythrina

indica, was once in Hawaii but has since disappeared. In order to

establish this, it will be requisite to show not only that the

extinction of a shore-plant is probable, but also to explain why the new

species has selected such arid inland localities for its stations, to

account for the loss of buoyancy of the seeds, and, if possible, to give

an instance of the production of a new species of Erythrina in a small

isolated oceanic island.

A study of the special circumstances of Hawaii leads one to conclude

that a shore-tree may become extinct in one of two ways. It may be

exterminated by insect pests, or it may be forced inland through

unsuitable coast-conditions and there be lost in the resulting new

species. One characteristic shore-tree, Cordia subcordata, has indeed

been almost exterminated by insects, and even Erythrina monosperma is

now from the same cause on its road to extinction (see Note 53); but

there is no indication of their leaving modified descendants behind that

are pest-proof. The most probable view then is that the littoral tree,

having been driven inland through the unsuitability of the

coast-conditions, such as lack of beaches or want of moisture, has there

become modified. This is what has really happened, as I have shown, with

Cæsalpinia bonducella in Hawaii. As indicated in Chapter XVII., this

characteristic beach-plant has here been driven off the beach. There

would thus be no difficulty in assigning a reason why a littoral tree

like Erythrina indica should select arid localities when it extends

inland, since, as is pointed out in Chapter IV. and in other parts of

this work, the plants of the beach and of the arid inland district

possess the same xerophilous habit.

With regard to the loss of buoyancy of the seeds in the case of

Erythrina monosperma, it may be remarked that this is precisely what has

happened with the seeds of Cæsalpinia bonducella, its usual associate on

the old lava-wastes in Hawaii, and with an inland species of Cæsalpinia

in Fiji. It is argued that the same thing has occurred with the inland

Hawaiian species of Canavalia and Sophora, as shown in later pages of

this chapter. It has certainly happened with the inland form of Afzelia

bijuga in Fiji, a tree dealt with in Chapter XVII. These are all

Leguminous genera; and in all of them, with the exception of Cæsalpinia,

where the floating power arises from a central cavity in the seed, the

seeds of the littoral species possess, like Erythrina indica, buoyant

kernels. Whilst most littoral plants with buoyant seeds or fruits retain

the floating capacity of the seed or fruit when they extend inland, the

Leguminosæ often offer exceptions to the rule.

That inland endemic species of Erythrina can be developed in isolated

islands is illustrated by the existence in Fernando Noronha, some two

hundred miles from the coast of Brazil, of a peculiar species, E.

aurantiaca, described by Mr. Ridley. Here also is found an inland

species of Guettarda peculiar to the locality; but in neither genus does

the littoral species occur.

Many difficulties will yet have to be explained before it can be finally

established that Erythrina monosperma has been derived from E. indica or

some similar shore species that was originally dispersed by the

currents; but we are almost driven towards such a view, since it is hard

to believe that the beans were carried to Hawaii by birds over some two

thousand miles of sea. Observers in other regions where littoral and

inland species of the genus occur may perhaps devote their attention to

the relation between the two; and if they are able to supplement

observation and experiment by a microscopical investigation, some

interesting results would be obtained. For instance, I would suggest

that in Queensland a thorough examination of the littoral E. indica and

the inland E. vespertilio might be undertaken; or perhaps there may be

some other littoral form.

With the two other Leguminous genera, Canavalia and Sophora, to be

immediately discussed, we have for the most part the same questions

raised. Both possess wide-ranging current-dispersed littoral species in

other parts of the Pacific, but only endemic inland species with

non-buoyant seeds in Hawaii. The pivot of the discussion will be here

also the impracticability of these inland species ever having reached

the Hawaiian Islands through the agency of the currents, and the great

difficulty in believing that their beans were carried unharmed by birds

over half the breadth of the Pacific Ocean. If we reject alike the

current, the bird, and the parentage of a lost littoral species, we must

fall back on the continental hypothesis, against which in the case of

Hawaii the evidence is overwhelming.

CANAVALIA.

This genus is represented in the tropical islands of the South Pacific

from Fiji to Tahiti by three littoral species, none of which have been

found in Hawaii, where only an endemic inland species exists. Reference

will alone be made here to such facts as bear on the probable history of

the mysterious Hawaiian species, additional particulars being given in

Note 54. The littoral species, Canavalia obtusifolia (D.C.), C. sericea

(Gray), and C. ensiformis (D.C.), have buoyant seeds and are dispersed

by the currents; whilst the inland Hawaiian species, C. galeata (Gaud.),

a forest climber peculiar to that group, has non-buoyant seeds. We thus

have repeated the problem of Erythrina monosperma. The absence of the

littoral species from Hawaii can scarcely be attributed to the failure

of the currents, since Ipomœa pes capræ, which accompanies C.

obtusifolia as a beach-creeper all round the tropical globe, is present

on the Hawaiian beaches. Nor can it arise from lack of floating-power on

the part of the seeds, since experiment indicates that the seeds of C.

obtusifolia will float for months unharmed in sea-water. Nor can it be

ascribed to climatic conditions, since this tropical shore species

extends into cooler latitudes than those of the Hawaiian Islands, being

found in the Kermadec Group and in the Bermudas, which are subtropical

both in position and as regards much of their vegetation. The reason

perhaps we may never learn from the plants themselves, though it may be

possible to obtain some light on the problem from outside sources.

Canavalia galeata differs much in its habits, as well as in some of its

characters, from the existing littoral species of regions outside the

Hawaiian Group. It is a stout climber ascending the forest trees to a

considerable height, though, as is indicated in Note 54, the shore

species sometimes display a tendency in the same direction. It is

described by Hillebrand as occurring “on all islands, in forests up to

2,000 feet.” Like those of the inland species of Erythrina (E.

monosperma), its seeds sink in sea-water even after being kept for four

years, nor could the pods be utilised for dispersal by the currents,

since they float, when unopened, only for four or five days. Here also,

as with Erythrina, the seeds of the inland species no longer possess the

buoyant kernels to which the floating capacity of the seeds of the coast

species is due. Though we have to exclude the currents, we can scarcely

in its case appeal to bird-agency when we wish to account for the

transportal of the original seeds to Hawaii, as that would imply that

birds can carry beans nearly an inch, or 2 to 2.5 centimetres, in length

unharmed in their stomachs over a tract of ocean some 1,500 or 2,000

miles across. We should have to learn much that is unexpected of the

modes of dispersal of the Leguminosæ before we could accept such an

hypothesis.

Canavalia galeata indeed presents to the student of dispersal one of the

enigmas of the Hawaiian flora; and it should be noted that the mystery

of its distribution is concerned not only with the means of transportal

of the seeds of the original species to the group, but also with its

present dispersal among the islands. It is, however, suggestive that Dr.

Hillebrand mentions two varieties, one of them found on Kauai, with

somewhat smaller seeds; so that some inter-island differentiation is

evidently in progress. No attempt is made here to connect this inland

species directly with the absent beach-plants. That is a matter for the

systematist; but we are not tied down to existing shore-plants in

finding an ancestor, since the common parent of the littoral and inland

species may have been a shore-plant dispersed by the currents.

MEZONEURON.

Another closely parallel instance, offering, from the standpoint of

dispersal, the same difficulties presented by Canavalia galeata, is to

be found in Mezoneuron kauaiensis (Hillebr.), a tall inland shrub also

peculiar to the group and belonging alike to the Leguminosæ. The

difficulties are so nearly identical that the same explanation will have

to cover both; but it is significant that with Mezoneuron there is no

littoral species to which we can appeal to extricate us from the

difficulty. Yet the genus is related to Cæsalpinia, and the species was

first described by Mann as C. kauaiensis, so that it may have once

possessed a littoral species that has ceased to exist as such. When we

come to discuss Cæsalpinia and Afzelia (Chapter XVII.) we shall obtain

from those genera many suggestions as to the probable past of Canavalia

galeata and Mezoneuron kauaiensis, two of the greatest riddles presented

by the Leguminosæ of Hawaii.

The flat seeds of this species of Mezoneuron measure about an inch (2·5

cm.), and seem most unsuitable for dispersal by birds over a wide extent

of ocean. Nor can we appeal to the currents, since my experiments in

Hawaii show that the seeds have no buoyancy and that the pods only float

for a week in sea-water. Dr. Hillebrand records this shrub from Kauai,

Oahu, and Maui; I found it also on the lower slopes of Hualalai in

Hawaii and therefore the same question of inter-island dispersal here

presents itself that was connected with Canavalia galeata, since we have

also to explain the transport of the seeds between islands 70 to 150

miles apart. The critical point in the history of these two enigmatic

inland plants of the Hawaiian Islands was doubtless concerned with the

loss of buoyancy of the seeds of the original littoral plant. It will

subsequently be shown that this is what is now in actual operation with

Cæsalpinia and Afzelia in different parts of the Pacific.

SOPHORA.

In this genus, as in Erythrina and Canavalia, we have a littoral

species, Sophora tomentosa, that ranges over the tropical beaches of the

globe, including most of the islands of the Pacific, but does not occur

in Hawaii, where the genus is represented by an endemic inland species,

S. chrysophylla. Here also we find the shore-species with seeds capable

of floating for months on account of their buoyant kernels, and the

inland species with seeds that sink even after years of drying (see Note

56). Unless other inland species of Sophora have recently been described

from the tropical Pacific, the Hawaiian species is the only one of its

kind known from this region.

But the problem wears a different aspect in the case of this genus,

since the endemic inland species of Hawaii is a tree of the mountains

where a temperate climate prevails, whilst Sophora tomentosa is a shrub

of the tropical beach that only at times extends into subtropical

latitudes. The Mamani tree, as the Hawaiians name S. chrysophylla,

extends up to 9,000 or 10,000 feet above the sea, forming, with Myoporum

sandwicense and one or two other trees and shrubs, the highest belt of

the forest in the larger islands. It is in the open woodland between

6,000 and 7,000 feet that it is most at home, and here it attains a

height of 20 to 30 feet. It descends in places to as low as 2,000 feet

above sea-level; but here is living under uncongenial conditions, and,

like Myoporum sandwicense, becomes dwarfed and shrubby. The climatic

conditions under which S. chrysophylla thrives in the Hawaiian mountains

are therefore those of the temperate zone. From the data given in

Chapter XIX., the mean annual temperature at an elevation of 6,000 to

7,000 feet would probably be about 55°, the average temperature of New

Zealand.

We must therefore look to the temperate and not to the tropical zone for

the home of the parent species of Sophora chrysophylla; and if it was

originally derived from a shore-plant dispersed by the currents, the

widespread S. tomentosa could scarcely have been the species concerned.

But this strand-plant is disqualified for another potent reason, since

it belongs to a different section of the genus. Whilst S. tomentosa

belongs to the section possessing smooth pods, S. chrysophylla is

referred to the section Edwardsia having four-winged pods, which

comprises about ten species found in Chile and Peru, Hawaii, New

Zealand, Further India, and the Isle of Bourbon. What strange principle

in distribution, we may fitly ask, has linked together in this odd

fashion the continents of the Old and New World and the islands of the

Indian and Pacific oceans?

Yet, discredited as Sophora tomentosa is as a possible parent of the

Hawaiian mountain species, it may yet afford us a clue. It is

significant that the distribution of this wide-ranging beach-shrub in

the tropics of the southern hemisphere is almost coterminous with that

of Sophora tetraptera, a species widely spread in the south temperate

zone from Chile to New Zealand and extending towards the tropics as far

as Juan Fernandez in lat. 33° S. and to Easter Island in lat. 27° S.

Though not strictly a beach-plant, S. tetraptera is a plant of the

sea-border; and it is remarkable, but not surprising, how in New

Zealand, one of its principal homes, its behaviour in respect of its

vertical distribution presents a great contrast to that of S.

chrysophylla in the tropical latitudes of Hawaii. We have seen that, in

Hawaii, S. chrysophylla, which thrives as a tree 20 to 30 feet high in

the mountains, becomes shrubby when it descends to the lower levels. In

New Zealand, S. tetraptera is, as we learn from Kirk, a prostrate shrub

in the mountains, whilst in the lower elevations towards the sea it

becomes a tree 30 and even 50 feet in height. It can scarcely be doubted

that, if we exchanged the habitats of these Hawaiian and New Zealand

species, each would to a great extent take up the other’s station and

the other’s habit.

The whole problem of the dispersal of Sophora was brought immediately to

my notice at Corral, in latitude 40° S. on the coast of Chile. Here a

small tree of the section Edwardsia was growing in fruit on the lower

slopes of the hills, becoming bushy when descending to the beach.

Specimens of its four-winged pods have been identified at the Kew Museum

as those of Sophora tetraptera; and, as far as the pod is concerned, I

cannot distinguish between my specimens of the Hawaiian S. chrysophylla

and the Chilian species. Subsequently I found the buoyant seeds of the

same plant amongst the stranded beach-drift at Bahia San Vincente,

nearly 200 miles further north. This led to my experimenting on the

capacity of the plant for dispersal by the currents, and as a result it

was ascertained (see Note 56) that whilst, as in the case of S.

chrysophylla, the pods floated only one or two weeks, the seeds on

account of their buoyant kernels floated for several months in

sea-water, retaining their power of germination. The Chilian plant thus

differs significantly in its capacity for dispersal by currents from the

Hawaiian species, the seeds of which sink in sea-water even after years

of drying.

The Mamani tree in Hawaii had always been an object of great interest to

me. I was attracted by the mystery surrounding its origin and had long

suspected that the clue was to be found in the non-buoyancy of its seeds

and in the absence of a littoral species of the genus. When in Fiji it

was to the littoral Sophora tomentosa that I looked in vain for a

solution of the riddle, and seven years afterwards on the coast of Chile

a solution of this enigma of the Hawaiian mountains presented itself in

the form of an argument somewhat in this shape.

On account of the elevated station of the Mamani tree (S. chrysophylla)

in Hawaii it is to be inferred that the original species was a plant of

the temperate regions or of the uplands of some tropical mountains. If

it has had its origin in some shore-plant dispersed by the currents,

that species can only now be found on the coasts of extra-tropical

regions. Such a maritime plant had buoyant seeds; and plants of this

type are presented by Sophora tetraptera and its allied species that are

at home in the cool latitudes of the southern hemisphere, as in Chile

and New Zealand. No difficulty, as I argued, could be connected with the

loss of buoyancy of the seeds of the Hawaiian mountain species, since it

follows the general principle (laid down in Chapter II.) that in the

same genus coast species have buoyant seeds or fruits, and inland

species those that sink; and in support of this view it was recalled

that this is what happens to the seeds of Cæsalpinia bonducella and

Afzelia bijuga when the plants extend inland in the Pacific islands. It

was held, in short, that the original form of Sophora chrysophylla in

Hawaii was a coast plant with buoyant seeds, and therefore indebted for

its presence to the currents. Hailing from an extra-tropical region, it

abandoned the beach and found suitable conditions of existence in the

mountains, where it underwent specific differentiation. Such was the

explanation that presented itself to me on a Chilian beach.

The first objection that offers itself against this view is that Sophora

chrysophylla is one of several species characterising the antarctic

element of the mountain flora of Hawaii, and that many of these plants,

such as those of the genera Astelia, Coprosma, Gunnera, Myoporum, &c.,

could only have reached these islands through the agency of frugivorous

birds (see Chapter XXIII.). There is, therefore, something to be said

for this mode of dispersal; but though one can understand how hard seeds

and the “stones” and crustaceous pyrenes of fleshy fruits might be

transported unharmed in a bird’s stomach half-way across the Pacific

Ocean to the distant group of Hawaii, it is difficult to understand how

Leguminous seeds, except in such cases as Tephrosia piscatoria, could be

ejected unharmed by a bird after an ocean passage of some 1,500 or 2,000

miles. Yet evidence pointing to such a possibility is not lacking. It

was pointed out by W. O. Focke (_Nat. schaft. Ver. zur Bremen_,

Abhandl., Band 5, 1876) that for many Leguminosæ we are driven to the

agency of birds in order to explain their dispersal. In this connection

he mentions the case of a pigeon killed by some beast of prey that he

found in his garden in the early winter. In the following spring he

noticed numerous seedlings of Vicia faba sprouting up from amongst the

feathers that alone remained of the bird. In this observation he

detected the normal method of the dispersal of the Leguminosæ by birds,

the seeds not being ejected by the bird but being set free by its death.

It is well known that Darwin had this idea in his mind when he conducted

his experiments on the dispersal of seeds; and reference may here be

made to one that is recorded in _More Letters of Charles Darwin_ (i.,

436). Out of a number of seeds left in the stomach of an eagle for

eighteen hours, the majority were killed; but amongst the few that

germinated afterwards was a seed of clover (Trifolium). If such a bird

had carried a Sophora seed to Hawaii, this would have involved a

continuous flight of, on the average, 100 miles per hour for a period of

fifteen to twenty hours. This would just come within the limitations

laid down by Gätke as regards length and velocity of flight—a subject

discussed in Chapter XXXIII.

We will now turn to the Sophora seeds themselves for evidence of their

capacity of surviving the perils of such a journey. The seeds of Sophora

chrysophylla, which are about a quarter of an inch (6 to 7 mm.) in

length, possess unusually hard coverings for the order, and in that

respect appear fitted for dispersal by animals. Indeed, in the large

island of Hawaii wild pigs and sheep feed on the pods, and no doubt aid

in the distribution of the plant over the island through the germination

of ejected undigested seeds. But since the species is found on most of

the larger islands, it is apparent that to birds we must look for the

explanation of its inter-island dispersal. Mr. Wilson, in his _Aves

Hawaienses_, remarks that one of the Hawaiian finches (Loxioides) feeds

on the seeds of this tree, which probably, he adds, also serve as the

food of Chloridops kona, another big finch; and it is to be inferred

from the observations of Mr. Perkins, quoted by Mr. Evans in his book on

Birds, that the Drepanididæ, a family peculiar to Hawaii, are in the

habit of splitting the pods of trees like Acacia koa and Sophora

chrysophylla to obtain the seeds. It would, however, seem that the

agency of birds confined to these islands does not carry us very far

when we wish to explain the original transport of the seeds over a

breadth of ocean of some 1,500 miles and more. Yet we know that this

must have happened with some of the Hawaiian plants, such as Osteomeles

anthyllidifolia and Nertera depressa, that are not confined to these

islands and possess fruits that would attract frugivorous birds. But

whether it has occurred with the dry beans of the Hawaiian species of

Sophora is another matter.

On the whole I am inclined to the view, bearing in mind the general

indications of the Leguminosæ in the Pacific, that S. chrysophylla

originally reached Hawaii as a littoral plant through the agency of the

currents. Many points still need investigation; but it may be pointed

out that South America probably received Sophora tetraptera from New

Zealand by the West Wind Drift Current.

OCHROSIA (Apocyneæ).

This genus seems to offer the strongest testimony in support of the

derivation of an inland species from a strand-plant. The drupes are so

large, the minimum size of the “stone” being 1-1/2 or 2 inches (37 to 50

mm.), and so dry and unattractive for birds, that any other agency but

that of the currents appears to be out of the question. Indeed their dry

appearance would suggest to my readers that only birds of the habits of

the ostrich would venture on such a diet. It is, however, worth noting

that whilst in the Keeling Islands I learned that a cassowary that had

been kept on the atoll was a very efficient distributor of the seeds of

Ochrosia parviflora, scattering the undigested stones everywhere, and

causing the young trees to become so numerous that they had to be

destroyed. A similar habit of the cassowary in the Aru Islands is

recorded by Beccari, where the dry fruits of a palm, 2-1/2 inches

across, are swallowed by these birds and the seeds dispersed.

Cassowaries are active agents in dissemination, for they swallow every

kind of pulpy fruit, and convey them long distances undigested; they are

also excellent swimmers and traverse considerable expanses of water

(Beccari, quoted in _Chall. Bot._, iv., 297, 313).

Modern ornithologists would probably not object to our appealing to the

former volant habits of the cassowary and its allies even across a wide

tract of sea; but, excepting in New Zealand and its vicinity, such birds

are not at our disposal in the island groups of the open Pacific. There

is a possibility that the extinct Columbæ and other exterminated birds

of the Mascarene Islands might account for some anomalies in their

floras; and in Chapter XVI. reference is made to the fact that these

islands possess more endemic species of Pandanus than any other oceanic

groups, a genus possessing drupes that in the case of inland species

seem unfit for any mode of dispersal with which we are familiar. In the

islands of the tropical Pacific, however, it is not possible to find

such a way out of the difficulty, since, as shown in Chapter XXXIII.,

the birds are lacking.

The genus, according to the _Index Kewensis_, includes about ten species

distributed over the islands of the Indian Ocean, and found also in

Malaya, Australia, and throughout the Pacific. It is essentially an

insular genus, and two at least of the species are wide-ranging littoral

trees, one, Ochrosia borbonica, mainly distributed over the islands of

the Indian Ocean and of Malaya, and the other, O. parviflora, chiefly of

the islands of the Pacific. It will be out of place to deal here in any

detail with this interesting genus, and my remarks will be confined to

such matters as concern the origin of the inland species of the Hawaiian

Islands, species that are peculiar to that group. Some confusion has

prevailed amongst different authors in the determination of the limits

of the various species, and to avoid this I have mainly followed

Schumann in his monograph on the order (Engler’s _Naturl. Pflanz. Fam._,

Theil 4, Abth. 2, 1895), as indicated in Note 57.