THE CO-EXISTENCE OF CLOSELY SIMILAR STRUCTURES OF DIVERSE ORIGIN.
Chances against concordant variations.--Examples of discordant
ones.--Concordant variations not unlikely on a non-Darwinian
evolutionary hypothesis.--Placental and implacental mammals.--Birds and
reptiles.--Independent origins of similar sense organs.--The ear.--The
eye.--Other coincidences.--Causes besides Natural Selection produce
concordant variations in certain geographical regions.--Causes besides
Natural Selection produce concordant variations in certain zoological
and botanical groups.--There are homologous parts not genetically
related.--Harmony in respect of the organic and inorganic
worlds.--Summary and conclusion.
The theory of "Natural Selection" supposes that the varied forms and
structure of animals and plants have been built up merely by indefinite,
fortuitous,[49] minute variations in every part and in all
directions--those variations only being preserved which are directly or
indirectly useful to the individual possessing them, or necessarily
correlated with such useful variations.
[Illustration: WINGBONES OF PTERODACTYLE, BAT, AND BIRD.
(_Copied, by permission, from Mr. Andrew Murray's "Geographical
Distribution of Mammals."_)]
On this theory the chances are almost infinitely great against the
independent, accidental occurrence and preservation of two similar series
of minute variations resulting in the independent development of two
closely similar forms. In all cases, no doubt (on this same theory), _some_
adaptation to habit or need would gradually be evolved, but that {64}
adaptation would surely be arrived at by different roads. The organic world
supplies us with multitudes of examples of similar functional results being
attained by the most diverse means. Thus the body is sustained in the air
by birds and by bats. In the first case it is so sustained by a limb in
which the bones of the hand are excessively reduced, but which is provided
with immense outgrowths from the skin--namely, the feathers of the wing. In
the second case, however, the body is sustained in the air by a limb in
which the bones of the hand are enormously increased in length, and so
sustain a great expanse of naked skin, which is the flying membrane of the
bat's wing. Certain fishes and certain reptiles can also flit and take very
prolonged jumps in the air. The flying-fish, however, takes these by means
of a great elongation of the rays of the pectoral fins--parts which cannot
be said to be of the same nature as the constituents of the wing of either
the bat or the bird. The little lizard, which enjoys the formidable name of
"flying-dragon," flits by means of a structure altogether peculiar--namely,
by the liberation and great elongation of some of the ribs which support a
fold of skin. In the extinct pterodactyles--which were _truly_ flying {65}
reptiles--we meet with an approximation to the structure of the bat, but in
the pterodactyle we have only one finger elongated in each hand: a striking
example of how the very same function may be provided for by a modification
similar in principle, yet surely manifesting the independence of its
origin. When we go to lower animals, we find flight produced by organs, as
the wings of insects, which are not even modified limbs at all; or we find
even the function sometimes subserved by quite artificial means, as in the
aërial spiders, which use their own threads to float with in the air. In
the vegetable kingdom the atmosphere is often made use of for the
scattering of seeds, by their being furnished with special structures of
very different kinds. The diverse modes by which such seeds are dispersed
are well expressed by Mr. Darwin. He says:[50] "Seeds are disseminated {66}
by their minuteness,--by their capsule being converted into a light
balloon-like envelope,--by being embedded in pulp or flesh, formed of the
most diverse parts, and rendered nutritious, as well as conspicuously
coloured, so as to attract and be devoured by birds,--by having hooks and
grapnels of many kinds and serrated awns, so as to adhere to the fur of
quadrupeds,--and by being furnished with wings and plumes, as different in
shape as elegant in structure, so as to be wafted by every breeze."
[Illustration: SKELETON OF THE FLYING-DRAGON.
(Showing the elongated ribs which support the flitting organ.)]
Again, if we consider the poisoning apparatus possessed by different
animals, we find in serpents a perforated--or rather very deeply
channelled--tooth. In wasps and bees the sting is formed of modified parts,
accessory in reproduction. In the scorpion, we have the median terminal
process of the body specially organized. In the spider, we have a specially
constructed antenna; and finally in the centipede a pair of modified
thoracic limbs.
[Illustration: A CENTIPEDE.]
It would be easy to produce a multitude of such instances of similar ends
being attained by dissimilar means, and it is here contended that by "the
action of Natural Selection" _only_ it is so improbable as to be
practically impossible for two exactly similar structures to have ever been
independently developed. It is so because the number of possible {67}
variations is indefinitely great, and it is therefore an indefinitely great
number to one against a similar series of variations occurring and being
similarly preserved in any two independent instances.
The difficulty here asserted applies, however, only to pure Darwinism,
which makes use _only_ of indirect modifications through the survival of
the fittest.
Other theories (for example, that of Mr. Herbert Spencer) admit the
_direct_ action of conditions upon animals and plants--in ways not yet
fully understood--there being conceived to be at the same time a certain
peculiar but limited power of response and adaptation in each animal and
plant so acted on. Such theories have not to contend against the difficulty
proposed, and it is here urged that even very complex extremely similar
structures have again and again been developed quite independently one of
the other, and this because the process has taken place not by merely
haphazard, indefinite variations in all directions, but by the concurrence
of some other and internal natural law or laws co-operating with external
influences and with Natural Selection in the evolution of organic forms.
It must never be forgotten that to admit any such constant operation of any
such unknown natural cause is to deny the purely Darwinian theory, which
relies upon the survival of the fittest by means of minute fortuitous
indefinite variations.
Amongst many other obligations which the Author has to acknowledge to
Professor Huxley, are the pointing out of this very difficulty, and the
calling his attention to the striking resemblance between certain teeth of
the dog and of the thylacine as one instance, and certain ornithic
peculiarities of pterodactyles as another.
Mammals[51] are divisible into one great group, which comprises the {68}
immense majority of kinds termed, from their mode of reproduction,
_placental Mammals_, and into another very much smaller group comprising
the pouched-beasts or marsupials (which are the kangaroos, bandicoots,
phalangers, &c., of Australia), and the true opossums of America, called
_implacental Mammals_. Now the placental mammals are subdivided into
various orders, amongst which are the flesh-eaters (Carnivora, _i.e._ cats,
dogs, otters, weasels, &c.), and the insect-eaters (Insectivora, _i.e._
moles, hedgehogs, shrew-mice, &c.). The marsupial mammals also present a
variety of forms (some of which are carnivorous beasts, whilst others are
insectivorous), so marked that it has been even proposed to divide them
into orders parallel to the orders of placental beasts.
The resemblance, indeed, is so striking as, on Darwinian principles, to
suggest the probability of genetic affinity; and it even led Professor
Huxley, in his Hunterian Lectures, in 1866, to promulgate the notion that a
vast and widely-diffused marsupial fauna may have existed anteriorly to the
development of the ordinary placental, non-pouched beasts, and that the
carnivorous, insectivorous, and herbivorous placentals may have
respectively descended from the carnivorous, insectivorous, and herbivorous
marsupials.
[Illustration: TEETH OF UROTRICHUS AND PERAMELES.]
Amongst other points Professor Huxley called attention to the resemblance
between the anterior molars of the placental dog with those of the
marsupial thylacine. These, indeed, are strikingly similar, but there are
better examples still of this sort of coincidence. Thus it has often {69}
been remarked that the insectivorous marsupials, _e.g. Perameles_,
wonderfully correspond, as to the form of certain of the grinding teeth,
with certain insectivorous placentals, _e.g. Urotrichus_.
Again, the saltatory insectivores of Africa (_Macroscelides_) not only
resemble the kangaroo family (_Macropodidæ_) in their jumping habits and
long hind legs, but also in the structure of their molar teeth, and even
further, as I have elsewhere[52] pointed out, in a certain similarity of
the upper cutting teeth, or incisors.
Now these correspondences are the more striking when we bear in mind that a
similar dentition is often put to very different uses. The food of
different kinds of apes is very different, yet how uniform is their dental
structure! Again, who, looking at the teeth of different kinds of bears,
would ever suspect that one kind was frugivorous, and another a devourer
exclusively of animal food?
The suggestion made by Professor Huxley was therefore one which had much to
recommend it to Darwinians, though it has not met with any notable
acceptance, and though he seems himself to have returned to the older
notion, namely, that the pouched-beasts, or marsupials, are a special
ancient offshoot from the great mammalian class.
But whichever view may be the correct one, we have in either case a number
of forms similarly modified in harmony with surrounding conditions, and
eloquently proclaiming some natural plastic power, other than mere
fortuitous variation with survival of the fittest. If, however, the Reader
thinks that teeth are parts peculiarly qualified for rapid variation (in
which view the Author cannot concur), he is requested to suspend his
judgment till he has considered the question of the independent evolution
of the _highest organs of sense_. If this seems to establish the {70}
existence of some other law than that of "Natural Selection," then the
operation of that other law may surely be also traced in the harmonious
co-ordinations of dental form.
The other difficulty, kindly suggested to me by the learned Professor,
refers to the structure of birds, and of extinct reptiles more or less
related to them.
The class of birds is one which is remarkably uniform in its organization.
So much is this the case, that the best mode of subdividing the class is a
problem of the greatest difficulty. Existing birds, however, present forms
which, though closely resembling in the greater part of their structure,
yet differ importantly the one from the other. One form is exemplified by
the ostrich, rhea, emeu, cassowary, apteryx, dinornis, &c. These are the
_struthious_ birds. All other existing birds belong to the second division,
and are called (from the keel on the breast-bone) _carinate_ birds.
Now birds and reptiles have such and so many points in common, that
Darwinians must regard the former as modified descendants of ancient
reptilian forms. But on Darwinian principles it is impossible that the
class of birds so uniform and homogeneous should have had a double
reptilian origin. If one set of birds sprang from one set of reptiles, and
another set of birds from another set of reptiles, the two sets could
never, by "Natural Selection" only, have grown into such a perfect
similarity. To admit such a phenomenon would be equivalent to abandoning
the theory of "Natural Selection" as the sole origin of species.
Now, until recently it has generally been supposed by evolutionists that
those ancient flying reptiles, the pterodactyles, or forms allied to them,
were the progenitors of the class of birds; and certain parts of their
structure especially support this view. Allusion is here made to the
bladebone (scapula), and the bone which passes down from the shoulder-joint
to the breast-bone (viz. the coracoid). These bones are such remarkable
anticipations of the same parts in ordinary (_i.e._ carinate) birds {71}
that it is hardly possible for a Darwinian not to regard the resemblance as
due to community of origin. This resemblance was carefully pointed out by
Professor Huxley in his "Hunterian Course" for 1867, when attention was
called to the existence in _Dimorphodon macronyx_ of even that small
process which in birds gives attachment to the upper end of the
merrythought. Also Mr. Seeley[53] has shown that in pterodactyles, as in
birds, the optic lobes of the brain were placed low down on each
side--"lateral and depressed." Nevertheless, the view has been put forward
and ably maintained by the same Professor,[54] as also by Professor Cope in
the United States, that the line of descent from reptiles to birds has not
been from ordinary reptiles, through pterodactyle-like forms, to ordinary
birds, but to the struthious ones from certain extinct reptiles termed
Dinosauria; one of the most familiarly known of which is the Iguanodon of
the Wealden formation. In these Dinosauria we find skeletal characters
unlike those of ordinary (_i.e._ carinate) birds, but closely resembling in
certain points the osseous structure of the struthious birds. Thus a
difficulty presents itself as to the explanation of the three following
relationships:--(1) That of the Pterodactyles with carinate birds; (2) that
of the Dinosauria with struthious birds; (3) that of the carinate and
struthious birds with each other.
Either birds must have had two distinct origins whence they grew to their
present conformity, or the very same skeletal, and probably cerebral
characters must have spontaneously and independently arisen. Here is a
dilemma, either horn of which bears a threatening aspect to the exclusive
supporter of "Natural Selection," and between which it seems somewhat {72}
difficult to choose.
It has been suggested to me that this difficulty may be evaded by
considering pterodactyles and carinate birds as independent branches from
one side of an ancient common trunk, while similarly the Dinosauria and
struthious birds are taken to be independent branches from the other side
of the same common trunk; the two kinds of birds resembling each other so
much on account of their later development from that trunk as compared with
the development of the reptilian forms. But to this it may be replied that
the ancient common stock could not have had at one and the same time a
shoulder structure of _both kinds_. It must have been that of the
struthious birds or that of the carinate birds, or something different from
both. If it was that of the struthious birds, how did the pterodactyles and
carinate birds independently arrive at the very same divergent structure?
If it was that of the carinate birds, how did the struthious birds and
Dinosauria independently agree to differ? Finally, if it was something
different from either, how did the carinate birds and pterodactyles take on
independently one special common structure when disagreeing in so many;
while the struthious birds, agreeing in many points with the Dinosauria,
agree yet more with the carinate birds? Indeed by no arrangement of
branches from a stem can the difficulty be evaded.
Professor Huxley seems inclined[55] to cut the Gordian knot by considering
the shoulder structure of the pterodactyle as independently educed, and
having relation to physiology only. This conception is one which harmonizes
completely with the views here advocated, and with those of Mr. Herbert
Spencer, who also calls in direct modification to the aid of "Natural
Selection." That merely minute, indefinite variations in all directions
should unaided have independently built up the shoulder structure of {73}
the pterodactyles and carinate birds, and have laterally depressed their
optic lobes, at a time so far back as the deposition of the Oolite
strata,[56] is a coincidence of the highest improbability; but that an
innate power and evolutionary law, aided by the corrective action of
"Natural Selection," should have furnished like needs with like aids, is
not at all improbable. The difficulty does not tell against the theory of
evolution, but only against the specially Darwinian form of it. Now this
form has never been expressly adopted by Professor Huxley; so far from it,
in his lecture on this subject at the Royal Institution before referred to,
he observes,[57] "I can testify, from personal experience, it is possible
to have a complete faith in the general doctrine of evolution, and yet to
hesitate in accepting the Nebular, or the Uniformitarian, or the Darwinian
hypotheses in all their integrity and fulness."
[Illustration: THE ARCHEOPTERYX (of the Oolite strata).]
It is quite consistent, then, in the Professor to explain the {74}
difficulty as he does; but it would not be similarly so with an absolute
and pure Darwinian.
Yet stronger arguments of an analogous kind are, however, to be derived
from the highest organs of sense. In the most perfectly organized
animals--those namely which, like ourselves, possess a spinal column--the
internal organs of hearing consist of two more or less complex membranous
sacs (containing calcareous particles--otoliths), which are primitively or
permanently lodged in two chambers, one on each side of the cartilaginous
skull. The primitive cartilaginous cranium supports and protects the base
of the brain, and the auditory nerves pass from that brain into the
cartilaginous chambers to reach the auditory sacs. These complex
arrangements of parts could not have been evolved by "Natural Selection,"
_i.e._ by minute accidental variations, except by the action of such
through a vast period of time; nevertheless, it was fully evolved at the
time of the deposition of the upper Silurian rocks.
Cuttle-fishes (_Cephalopoda_) are animals belonging to the molluscous
primary division of the animal kingdom, which division contains animals
formed upon a type of structure utterly remote from that on which the
animals of the higher division provided with a spinal column are
constructed. And indeed no transitional form (tending even to bridge over
the chasm between these two groups) has ever yet been discovered, either
living or in a fossilized condition.[58]
Nevertheless, in the two-gilled Cephalopods (_Dibranchiata_) we find the
brain supported and protected by a cartilaginous cranium. In the base of
this cranium are two cartilaginous chambers. In each chamber is a
membranous sac containing an otolith, and the auditory nerves pass from the
cerebral ganglia into the cartilaginous chambers to reach the auditory
sacs. Moreover, it has been suggested by Professor Owen that {75}
sinuosities between processes projecting from the inner wall of each
chamber "seem to be the first rudiments of those which, in the higher
classes (_i.e._ in animals with a spinal column), are extended in the form
of canals and spiral chambers, within the substance of the dense nidus of
the labyrinth."[59]
[Illustration: CUTTLE-FISH.
A. Ventral aspect. B. Dorsal aspect.]
Here, then, we have a wonderful coincidence indeed; two highly complex
auditory organs, marvellously similar in structure, but which must
nevertheless have been developed in entire and complete independence one of
the other! It would be difficult to calculate the odds against the
independent occurrence and conservation of two such complex series of
merely accidental and minute haphazard variations. And it can never be {76}
maintained that the sense of hearing could not be efficiently subserved
otherwise than by such sacs, in cranial cartilaginous capsules so situated
in relation to the brain, &c.
Our wonder, moreover, may be increased when we recollect that the
two-gilled cephalopods have not yet been found below the lias, where they
at once abound; whereas the four-gilled cephalopods are Silurian forms.
Moreover, the absence is in this case significant in spite of the
imperfection of the geological record, because when we consider how many
individuals of various kinds of four-gilled cephalopods have been found, it
is fair to infer that at the least a certain small percentage of dibranchs
would also have left traces of their presence had they existed. Thus it is
probable that some four-gilled form was the progenitor of the dibranch
cephalopods. Now the four-gilled kinds (judging from the only existing
form, the nautilus) had the auditory organ in a very inferior condition of
development to what we find in the dibranch; thus we have not only evidence
of the independent high development of the organ in the former, but also
evidence pointing towards a certain degree of comparative rapidity in its
development.
Such being the case with regard to the organ of hearing, we have another
yet stronger argument with regard to the organ of sight, as has been well
pointed out by Mr. J. J. Murphy.[60] He calls attention to the fact that
the eye must have been perfected in at least "three distinct lines of
descent," alluding not only to the molluscous division of the animal
kingdom, and the division provided with a spinal column, but also to a
third primary division, namely, that which includes all insects, spiders,
crabs, &c., which are spoken of as Annulosa, and the type of whose
structure is as distinct from that of the molluscous type on the one hand,
as it is from that of the type with a spinal column (_i.e._ the vertebrate
type) on the other.
{77}
In the cuttle-fishes we find an eye even more completely constructed on the
vertebrate type than is the ear. Sclerotic, retina, choroid, vitreous
humour, lens, aqueous humour, all are present. The correspondence is
wonderfully complete, and there can hardly be any hesitation in saying that
for such an exact, prolonged, and correlated series of similar structures
to have been brought about in two independent instances by merely
indefinite and minute accidental variations, is an improbability which
amounts practically to impossibility. Moreover, we have here again the same
imperfection of the four-gilled cephalopod, as compared with the
two-gilled, and therefore (if the latter proceeded from the former) a
similar indication of a certain comparative rapidity of development.
Finally, and this is perhaps one of the most curious circumstances, the
process of formation appears to have been, at least in some respects, the
same in the eyes of these molluscous animals as in the eyes of vertebrates.
For in these latter the cornea is at first perforated, while different
degrees of perforation of the same part are presented by different adult
cuttle-fishes--large in the calamaries, smaller in the octopods, and
reduced to a minute foramen in the true cuttle-fish sepia.
Some may be disposed to object that the conditions requisite for effecting
vision are so rigid that similar results in all cases must be independently
arrived at. But to this objection it may well be replied that Nature
herself has demonstrated that there is no such necessity as to the details
of the process. For in the higher Annulosa, such as the dragon-fly, we meet
with an eye of an unquestionably very high degree of efficiency, but formed
on a type of structure only remotely comparable with that of the fish or
the cephalopod. The last-named animal might have had an eye as efficient as
that of a vertebrate, but formed on a distinct type, instead of being
another edition, as it were, of the very same structure.
In the beginning of this chapter examples have been given of the very {78}
diverse mode in which similar results have in many instances been arrived
at; on the other hand, we have in the fish and the cephalopod not only the
eye, but at one and the same time the ear also similarly evolved, yet with
complete independence.
Thus it is here contended that the similar and complex structures of both
the highest organs of sense, as developed in the vertebrates on the one
hand, and in the mollusks on the other, present us with residuary phenomena
for which "Natural Selection" alone is quite incompetent to account. And
that these same phenomena must therefore be considered as conclusive
evidence for the action of some other natural law or laws conditioning the
simultaneous and independent evolution of these harmonious and concordant
adaptations.
Provided with this evidence, it may be now profitable to enumerate other
correspondences, which are not perhaps in themselves inexplicable by
Natural Selection, but which are more readily to be explained by the action
of the unknown law or laws referred to--which action, as its necessity has
been demonstrated in one case, becomes _a priori_ probable in the others.
[Illustration: SKELETON OF AN ICHTHYOSAURUS.]
Thus the great oceanic Mammalia--the whales--show striking resemblances to
those prodigious, extinct, marine reptiles, the Ichthyosauria, and this not
only in structures readily referable to similarity of habit, but in such
matters as greatly elongated premaxillary bones, together with the
concealment of certain bones of the skull by other cranial bones. [Page 79]
Again, the aërial mammals, the bats, resemble those flying reptiles of the
secondary epoch, the pterodactyles; not only to a certain extent in the
breast-bone and mode of supporting the flying membrane, but also in the
proportions of different parts of the spinal column and the hinder (pelvic)
limbs.
Also bivalve shell-fish (_i.e._ creatures of the mussel, cockle, and oyster
class, which receive their name from the body being protected by a double
shell, one valve of which is placed on each side) have their two shells
united by one or two powerful muscles, which pass directly across from one
shell to the other, and which are termed "adductor muscles" because by
their contraction they bring together the valves and so close the shell.
[Illustration: CYTHERIDEA TOROSA.
[An ostracod (Crustacean), externally like a bivalve shell-fish
(Lamellibranch).]
Now there are certain animals which belong to the crab and lobster class
(Crustacea)--a class constructed on an utterly different type from that on
which the bivalve shell-fish are constructed--which present a very curious
approximation to both the form and, in a certain respect, the structure of
true bivalves. Allusion is here made to certain small Crustacea--certain
phyllopods and ostracods--which have the hard outer coat of their thorax so
modified as to look wonderfully like a bivalve shell, although its {80}
nature and composition are quite different. But this is by no means
all,--not only is there this external resemblance between the thoracic
armour of the crustacean and the bivalve shell, but the two sides of the
ostracod and phyllopod thorax are connected together also by an adductor
muscle!
[Illustration: A POLYZOON WITH BIRD'S-HEAD PROCESSES.]
{81}
The pedicellariæ of the echinus have been already spoken of, and the
difficulty as to their origin from minute, fortuitous, indefinite
variations has been stated. But structures essentially similar (called
avicularia, or "bird's-head processes") are developed from the surface of
the compound masses of certain of the highest of the polyp-like animals
(viz. the Polyzoa or, as they are sometimes called, the Bryozoa).
These compound animals have scattered over the surface of their bodies
minute processes, each of which is like the head of a bird, with an upper
and lower beak, the whole supported on a slender neck. The beak opens and
shuts at intervals, like the jaws of the pedicellariæ of the echinus, and
there is altogether, in general principle, a remarkable similarity between
the structures. Yet the echinus can have, at the best, none but the most
distant genetic relationship with the Polyzoa. We have here again therefore
complex and similar organs of diverse and independent origin.
[Illustration: BIRD'S-HEAD PROCESSES VERY GREATLY ENLARGED.]
In the highest class of animals (the Mammalia) we have almost always a
placental mode of reproduction, _i.e._ the blood of the foetus is placed in
nutritive relation with the blood of the mother by means of vascular
prominences. No trace of such a structure exists in any bird or in any
reptile, and yet it crops out again in certain sharks. There indeed it
might well be supposed to end, but, marvellous as it seems, it reappears in
very lowly creatures; namely, in certain of the ascidians, sometimes called
tunicaries or sea-squirts. [Page 82]
Now, if we were to concede that the ascidians were the common ancestors[61]
of both these sharks and of the higher mammals, we should be little, if
any, nearer to an explanation of the phenomenon by means of "Natural
Selection," for in the sharks in question the vascular prominences are
developed from one foetal structure (the umbilical vesicle), while in the
higher mammals they are developed from quite another part, viz. the
allantois.
[Illustration: Upper Figure--ANTECHINUS MINUTISSIMUS (_implacental_).
Lower Figure--MUS DELICATULUS (_placental_).]
So great, however, is the number of similar, but apparently independent,
structures, that we suffer from a perfect _embarras de richesses_. Thus,
for example, we have the convoluted windpipe of the sloth, reminding us{83}
of the condition of the windpipe in birds; and in another mammal, allied to
the sloth, namely the great ant-eater (Myrmecophaga), we have again an
ornithic character in its horny gizzard-like stomach. In man and the
highest apes the cæcum has a vermiform appendix, as it has also in the
wombat!
Also the similar forms presented by the crowns of the teeth in some seals,
in certain sharks, and in some extinct Cetacea may be referred to; as also
the similarity of the beak in birds, some reptiles, in the tadpole, and
cuttle-fishes. As to entire external form, may be adduced the wonderful
similarity between a true mouse (_Mus delicatulus_) and a small marsupial,
pointed out by Mr. Andrew Murray in his work on the "Geographical
Distribution of Mammals," p. 53, and represented in the frontispiece by
figures copied from Gould's "Mammals of Australia;" but instances enough
for the present purpose have been already quoted.
Additional reasons for believing that similarity of structure is produced
by other causes than merely by "Natural Selection" are furnished by certain
facts of zoological geography, and by a similarity in the mode of variation
being sometimes extended to several species of a genus, or even to widely
different groups; while the restriction and the limitation of such
similarity are often not less remarkable. Thus Mr. Wallace says,[62] as to
local influence: "Larger or smaller districts, or even single islands, give
a special character to the majority of their Papilionidæ. For instance:--1.
The species of the Indian region (Sumatra, Java, and Borneo) are almost
invariably smaller than the allied species inhabiting Celebes and the
Moluccas. 2. The species of New Guinea and Australia are also, though in a
less degree, smaller than the nearest species or varieties of the Moluccas.
3. In the Moluccas themselves the species of Amboyna are the largest. 4.
The species of Celebes equal or even surpass in size those of Amboyna. {84}
5. The species and varieties of Celebes possess a striking character in the
form of the anterior wings, different from that of the allied species and
varieties of all the surrounding islands. 6. Tailed species in India or the
Indian region become tailless as they spread eastward through the
Archipelago. 7. In Amboyna and Ceram the females of several species are
dull-coloured, while in the adjacent islands they are more brilliant."
Again:[63] "In Amboyna and Ceram the female of the large and handsome
_Ornithoptera Helena_ has the large patch on the hind wings constantly of a
pale dull ochre or buff colour; while in the scarcely distinguishable
varieties from the adjacent islands, of Bouru and New Guinea, it is of a
golden yellow, hardly inferior in brilliancy to its colour in the male sex.
The female of _Ornithoptera Priamus_ (inhabiting Amboyna and Ceram
exclusively) is of a pale dusky brown tint, while in all the allied species
the same sex is nearly black, with contracted white markings. As a third
example, the female of _Papilio Ulysses_ has the blue colour obscured by
dull and dusky tints, while in the closely allied species from the
surrounding islands, the females are of almost as brilliant an azure blue
as the males. A parallel case to this is the occurrence, in the small
islands of Goram, Matabello, Ké, and Aru, of several distinct species of
Euploea and Diadema, having broad bands or patches of white, which do not
exist in any of the allied species from the larger islands. These facts
seem to indicate some local influence in modifying colour, as
unintelligible and almost as remarkable as that which has resulted in the
modifications of form previously described."
After endeavouring to explain some of the facts in a way to be noticed
directly, Mr. Wallace adds:[64] "But even the conjectural explanation now
given fails us in the other cases of local modification. Why the species of
the Western Islands should be smaller than those further east; why those of
Amboyna should exceed in size those of Gilolo and New Guinea; why the {85}
tailed species of India should begin to lose that appendage in the islands,
and retain no trace of it on the borders of the Pacific; and why, in three
separate cases, the females of Amboyna species should be less gaily attired
than the corresponding females of the surrounding islands, are questions
which we cannot at present attempt to answer. That they depend, however, on
some general principle is certain, because analogous facts have been
observed in other parts of the world. Mr. Bates informs me that, in three
distinct groups, Papilios, which, on the Upper Amazon, and in most other
parts of South America, have spotless upper wings, obtain pale or white
spots at Pará and on the Lower Amazon, and also that the Æneas group of
Papilios never have tails in the equatorial regions and the Amazon valley,
but gradually acquire tails in many cases as they range towards the
northern or southern tropic. Even in Europe we have somewhat similar facts,
for the species and varieties of butterflies peculiar to the Island of
Sardinia are generally smaller and more deeply coloured than those of the
mainland, and the same has been recently shown to be the case with the
common tortoiseshell butterfly in the Isle of Man; while _Papilio
Hospiton_, peculiar to the former island, has lost the tail, which is a
prominent feature of the closely allied _P. Machaon_.
"Facts of a similar nature to those now brought forward would no doubt be
found to occur in other groups of insects, were local faunas carefully
studied in relation to those of the surrounding countries; and they seem to
indicate that climate and other physical causes have, in some cases, a very
powerful effect in modifying specific form and colour, and thus directly
aid in producing the endless variety of nature."
[Illustration: OUTLINES OF WINGS OF BUTTERFLIES OF CELEBES COMPARED WITH
THOSE OF ALLIED SPECIES ELSEWHERE.
Outer outline, _Papilio gigon_, of Celebes. Inner outline, _P. demolion_,
of Singapore and Java.--2. Outer outline, _P. miletus_, of Celebes. Inner
outline, _P. sarpedon_, India.--3. Outer outline, _Tachyris zarinda_,
Celebes. Inner outline, _T. nero_.]
With regard to butterflies of Celebes belonging to different families, they
present "a peculiarity of outline which distinguishes them at a glance from
those of any other part of the world:"[65] it is that the upper wings {86}
are generally more elongated and the anterior margin more curved. Moreover,
there is, in most instances, near the base an abrupt bend or elbow, which
in some species is very conspicuous. Mr. Wallace endeavours to explain {87}
this phenomenon by the supposed presence at some time of special
persecutors of the modified forms, supporting the opinion by the remark
that small, obscure, very rapidly flying and mimicked kinds have not had
the wing modified. Such an enemy occasioning increased powers of flight, or
rapidity in turning, he adds, "one would naturally suppose to be an
insectivorous bird; but it is a remarkable fact that most of the genera of
fly-catchers of Borneo and Java on the one side, and of the Moluccas on the
other, are almost entirely absent from Celebes. Their place seems to be
supplied by the caterpillar-catchers, of which six or seven species are
known from Celebes, and are very numerous in individuals. We have no
positive evidence that these birds pursue butterflies on the wing, but it
is highly probable that they do so when other food is scarce. Mr. Bates
suggested to me that the larger dragon-flies prey upon butterflies, but I
did not notice that they were more abundant in Celebes than elsewhere."[66]
Now, every opinion or conjecture of Mr. Wallace is worthy of respectful and
attentive consideration, but the explanation suggested and before referred
to hardly seems a satisfactory one. What the past fauna of Celebes may have
been is as yet conjectural. Mr. Wallace tells us that now there is a
remarkable _scarcity_ of fly-catchers, and that their place is supplied by
birds of which it can only be said that it is "highly probable" that they
chase butterflies "when other food is scarce." The quick eye of Mr. Wallace
failed to detect them in the act, as also to note any unusual abundance of
other insectivorous forms, which therefore, considering Mr. Wallace's zeal
and powers of observation, we may conclude do not exist. Moreover, even if
there ever has been an abundance of such, it is by no means certain that
they would have succeeded in producing the conformation in question, for
the effect of this peculiar curvature on flight is by no means clear. We
have here, then, a structure hypothetically explained by an uncertain {88}
property induced by a cause the presence of which is only conjectural.
Surely it is not unreasonable to class this instance with the others before
given, in which a common modification of form or colour coexists with a
certain geographical distribution quite independently of the destructive
agencies of animals. If physical causes connected with locality can
abbreviate or annihilate the tails of certain butterflies, why may not
similar causes produce an elbow-like prominence on the wings of other
butterflies? There are many such instances of simultaneous modification.
Mr. Darwin himself[67] quotes Mr. Gould as believing that birds of the same
species are more brightly coloured under a clear atmosphere, than when
living on islands or near the coast. Mr. Darwin also informs us that
Wollaston is convinced that residence near the sea affects the colour of
insects; and finally, that Moquin-Tandon gives a list of plants which, when
growing near the sea-shore, have their leaves in some degree fleshy, though
not so elsewhere. In his work on "Animals and Plants under
Domestication,"[68] Mr. Darwin refers to M. Costa as having (in _Bull. de
la Soc. Imp. d'Acclimat_. tome viii. p. 351) stated "that young shells
taken from the shores of England and placed in the Mediterranean at once
altered their manner of growth, and formed prominent diverging rays _like
those on the shells of the proper Mediterranean oyster_;" also to Mr.
Meehan, as stating (_Proc. Acad. Nat. Sc. of Philadelphia_, Jan. 28, 1862)
"that twenty-nine kinds of American trees all differ from their nearest
European allies in _a similar manner_, leaves less toothed, buds and seeds
smaller, fewer branchlets," &c. These are striking examples indeed!
But cases of simultaneous and similar modifications abound on all sides.
Even as regards our own species there is a very generally admitted opinion
that a new type has been developed in the United States, and this in about
a couple of centuries only, and in a vast multitude of individuals of {89}
diverse ancestry. The instances here given, however, must suffice, though
more could easily be added.
[Illustration: THE GREAT SHIELDED GRASSHOPPER.]
It may be well now to turn to groups presenting similar variations, not
through, but independently of, geographical distribution, and, as far as we
know, independently of conditions other than some peculiar nature and
tendency (as yet unexplained) common to members of such groups, which
nature and tendency seem to induce them to vary in certain definite lines
or directions which are different in different groups. Thus with regard to
the group of insects, of which the walking leaf is a member, Mr. Wallace
observes:[69] "The _whole family_[70] of the Phasmidæ, or spectres, to
which this insect belongs, is more or less imitative, and a great number of
the species are called 'walking-stick insects,' from their singular {90}
resemblance to twigs and branches."
[Illustration: THE SIX-SHAFTED BIRD OF PARADISE.]
Again, Mr. Wallace[71] tells us of no less than four kinds of orioles,
which birds mimic, more or less, four species of a genus of honey-suckers,
the weak orioles finding their profit in being mistaken by certain birds of
prey for the strong, active, and gregarious honey-suckers. Now, many other
birds would be benefited by similar mimicry, which is none the less
confined, in this part of the world, to the oriole genus. It is true that
the absence of mimicry in other forms may be explained by their possessing
some other (as yet unobserved) means of preservation. But it is
nevertheless remarkable, not so much that one species should mimic, as that
no less than four should do so in different ways and degrees, all these{91}
four belonging to _one and the same genus_.
[Illustration: THE LONG-TAILED BIRD OF PARADISE.]
In other cases, however, there is not even the help of protective action to
account for the phenomenon. Thus we have the wonderful birds of
Paradise,[72] which agree in developing plumage unequalled in beauty, but a
beauty which, as to details, is of different kinds, and produced in
different ways in different species. To develop "beauty and singularity of
plumage" is a character of the group, but not of any one definite kind, to
be explained merely by inheritance.
{92}
[Illustration]
Again, we have the very curious horned flies,[73] which agree indeed in a
common peculiarity, but in one singularly different in detail, in different
species and not known to have any protecting effect.
Amongst plants, also, we meet with the same peculiarity. The great group of
Orchids presents a number of species which offer strange and bizarre {93}
approximations to different animal forms, and which have often the
appearance of cases of mimicry, as it were in an incipient stage.
[Illustration: HORNED FLIES.]
[Illustration: THE MAGNIFICENT BIRD OF PARADISE.]
The number of similar instances which could be brought forward from amongst
animals and plants is very great, but the examples given are, it is {94}
hoped, amply sufficient to point towards the conclusion which other facts
will, it is thought, establish, viz. that there are causes operating (in
the evocation of these harmonious diverging resemblances) other than
"Natural Selection," or heredity, and other even than merely geographical,
climatal, or any simply external conditions.
Many cases have been adduced of striking likenesses between different
animals, not due to inheritance; but this should be the less surprising, in
that the very same individual presents us with likenesses between different
parts of its body (_e.g._, between the several joints of the backbone),
which are certainly not so explicable. This, however, leads to a rather
large subject, which will be spoken of in the eighth chapter of the present
work. Here it will be enough to affirm (leaving the proof of the assertion
till later) that parts are often homologous which have no direct genetic
relationship,--a fact which harmonizes well with the other facts here
given, but which "Natural Selection," pure and simple, seems unable to
explain.
But surely the independent appearance of similar organic forms is what we
might expect, _a priori_, from the independent appearance of similar
inorganic ones. As Mr. G. H. Lewes well observes,[74] "We do not suppose
the carbonates and phosphates found in various parts of the globe--we do
not suppose that the families of alkaloids and salts have any nearer
kinship than that which consists in the similarity of their elements, and
the conditions of their combination. Hence, in organisms, as in salts,
morphological identity may be due to a community of causal connexion,
rather than community of descent.
"Mr. Darwin justly holds it to be incredible that individuals identically
the same should have been produced through Natural Selection from parents
_specifically distinct_, but he will not deny that identical forms may
issue from parents _genetically distinct_, when these parent forms and {95}
the conditions of production are identical. To deny this would be to deny
the law of causation."
Professor Huxley has, however, suggested[75] that such mineral identity may
be explained by applying also to minerals a law of descent; that is, by
considering such similar forms as the descendants of atoms which inhabited
one special part of the primitive nebular cosmos, each considerable space
of which may be supposed to have been under the influence of somewhat
different conditions.
Surely, however, there can be no real parity between the relationship of
existing minerals to nebular atoms, and the relationship of existing
animals and plants to the earliest organisms. In the first place, the
latter have produced others by generative multiplication, which mineral
atoms never did. In the second, existing animals and plants spring from the
living tissues of preceding animals and plants, while existing minerals
spring from the chemical affinity of separate elements. Carbonate of soda
is not formed, by a process of reproduction, from other carbonate of soda,
but directly by the suitable juxtaposition of carbon, oxygen, and sodium.
Instead of approximating animals and minerals in the mode suggested, it may
be that they are to be approximated in quite a contrary fashion; namely, by
attributing to mineral species an internal innate power. For, as we must
attribute to each elementary atom an innate power and tendency to form
(under the requisite external conditions) certain unions with other atoms,
so we may attribute to certain mineral species--as crystals--an innate
power and tendency to exhibit (the proper conditions being supplied) a
definite and symmetrical external form. The distinction between animals and
vegetables on the one hand, and minerals on the other, is that, while in
the organic world close similarity is the result sometimes of inheritance,
sometimes of direct production independently of parental action, in the{96}
inorganic world the latter is the constant and only mode in which such
similarity is produced.
When we come to consider the relations of species to space--in other words,
the geographical distribution of organisms--it will be necessary to return
somewhat to the subject of the independent origin of closely similar forms,
in regard to which some additional remarks will be found towards the end of
the seventh chapter.
In this third chapter an effort has been made to show that while on the
Darwinian theory concordant variations are extremely improbable, yet Nature
presents us with abundant examples of such; the most striking of which are,
perhaps, the higher organs of sense. Also that an important influence is
exercised by conditions connected with geographical distribution, but that
a deeper-seated influence is at work, which is hinted at by those special
tendencies in definite directions, which are the properties of certain
groups. Finally, that these facts, when taken together, afford strong
evidence that "Natural Selection" has not been the exclusive or predominant
cause of the various organic structural peculiarities. This conclusion has
also been re-enforced by the consideration of phenomena presented to us by
the inorganic world. [Page 97]
* * * * *