THE ARGUMENTS FROM DISTRIBUTION.
§ 137. In §§ 105 and 106, we contemplated the phenomena of distribution in
Space. The general conclusions reached, in great part based on the evidence
brought together by Mr. Darwin, were that, "on the one hand, we have
similarly-conditioned, and sometimes nearly-adjacent, areas, occupied by
quite different Faunas. On the other hand, we have areas remote from each
other in latitude, and contrasted in soil as well as climate, which are
occupied by closely-allied Faunas." Whence it was inferred that "as like
organisms are not universally, or even generally, found in like habitats;
nor very unlike organisms, in very unlike habitats; there is no manifest
pre-determined adaptation of the organisms to the habitats." In other
words, the facts of distribution in Space do not conform to the hypothesis
of design. At the same time we saw that "the similar areas peopled by
dissimilar forms, are those between which there are impassable barriers;
while the dissimilar areas peopled by similar forms, are those between
which there are no such barriers;" and these generalizations appeared to
harmonize with the abundantly-illustrated truth, "that each species of
organism tends ever to expand its sphere of existence--to intrude on other
areas, other modes of life, other media."
By way of showing still more clearly the effects of competition among races
of organisms, let me here add some recently-published instances of the
usurpations of areas, and changes of distribution hence resulting. In the
_Natural History Review_ for January, 1864, Dr. Hooker quotes as follows
from some New Zealand naturalists:--"You would be surprised at the rapid
spread of European and other foreign plants in this country. All along the
sides of the main lines of road through the plains, a _Polygonum_
(_aviculare_), called 'Cow Grass,' grows most luxuriantly, the roots
sometimes two feet in depth, and the plants spreading over an area from
four to five feet in diameter. The dock (_Rumex obtusifolius_ or _R.
crispus_) is to be found in every river bed, extending into the valleys of
the mountain rivers, until these become mere torrents. The sow-thistle is
spread all over the country, growing luxuriantly nearly up to 6000 feet.
The water-cress increases in our still rivers to such an extent, as to
threaten to choke them altogether ... I have measured stems twelve feet
long and three-quarters of an inch in diameter. In some of the mountain
districts, where the soil is loose, the white clover is completely
displacing the native grasses, forming a close sward.... In fact, the young
native vegetation appears to shrink from competition with these more
vigorous intruders." "The native (Maori) saying is 'as the white man's rat
has driven away the native rat, so the European fly drives away our own,
and the clover kills our fern, so will the Maoris disappear before the
white man himself.'"
Given this universal tendency of the superior to overrun the habitats of
the inferior,[50] let us consider what, on the hypothesis of evolution,
will be the effects on the geographical relationships of species.
§ 138. A race of organisms cannot expand its sphere of existence without
subjecting itself to new external conditions. Those of its members which
spread over adjacent areas, inevitably come in contact with circumstances
partially different from their previous circumstances; and such of them as
adopt the habits of other organisms, necessarily experience re-actions more
or less contrasted with the re-actions before experienced. Now if changes
of organic structure are caused, directly or indirectly, by changes in the
incidence of forces; there must result unlikenesses of structure between
the divisions of a race which colonizes new habitats. Hence, in the absence
of obstacles to migration, we may anticipate manifest kinships between the
animals and plants of one area, and those of areas adjoining it. This
inference corresponds with an induction before set down (§ 106). In
addition to illustrations of it already quoted from Mr. Darwin, his pages
furnish others. One is that species which inhabit islands are allied to
species which inhabit neighbouring main lands; and another is that the
faunas of clustered islands show marked similarities. "Thus the several
islands of the Galapagos Archipelago are tenanted," says Mr. Darwin, "in a
quite marvellous manner, by very closely related species; so that the
inhabitants of each separate island, though mostly distinct, are related in
an incomparably closer degree to each other than to the inhabitants of any
other part of the world." Mr. Wallace has traced "variation as specially
influenced by locality" among the _Papilionidæ_ inhabiting the East Indian
Archipelago: showing how "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;" and
how "tailed species in India and the western islands lose their tails as
they spread eastward through the archipelago." During his travels on the
Upper Amazons, Mr. Bates found that "the greater part of the species of
_Ithomiæ_ changed from one locality to another, not further removed than
100 to 200 miles;" that "many of these local species have the appearance of
being geographical varieties;" and that in some species "most of the local
varieties are connected with their parent form by individuals exhibiting
all the shades of variation."
Further general relationships are to be inferred. If races of organisms,
ever being thrust by pressure of population into new habitats, undergo
modifications of structure as they diverge more and more widely in Space,
it follows that, speaking generally, the widest divergences in Space will
indicate the longest periods during which the descendants from a common
stock have been subject to modifying conditions; and hence that, among
organisms of the same group, the smaller contrasts of structure will be
limited to the smaller areas. This we find: "varieties being," as Dr.
Hooker says in his _Flora of Tasmania_, "more restricted in locality than
species, and these again than genera." Again, if races of organisms spread,
and as they spread are altered by changing incident forces; it follows that
where the incident forces vary greatly within given areas, the alterations
will be more numerous than in equal areas which are less-variously
conditioned. This, too, proves to be the fact. Dr. Hooker points out that
the relatively uniform regions have the fewest species; while in the most
multiform regions the species are the most numerous.
§ 139. Let us consider next, how the hypothesis of evolution corresponds
with the facts of distribution, not over different areas but through
different media. If all forms of organisms have descended from some
primordial form, it follows that since this primordial form must have
inhabited some one medium out of the several media now inhabited, the
peopling of other media by its descendants implies migration from one
medium to others--implies adaptations to media quite unlike the original
medium. To speak specifically--water being the medium in which the lowest
living forms exist, the implication is that the earth and the air have been
colonized from the water. Great difficulties appear to stand in the way of
this assumption. Ridiculing those who alleged the uniserial development of
organic forms, who, indeed, laid themselves open to ridicule by their many
untenable propositions, Von Baer writes--"A fish, swimming towards the
shore desires to take a walk, but finds his fins useless. They diminish in
breadth for want of use, and at the same time elongate. This goes on with
children and grandchildren for a few millions of years, and at last who can
be astonished that the fins become feet? It is still more natural that the
fish in the meadow, finding no water, should gape after air, thereby, in a
like period of time developing lungs; the only difficulty being that in the
meanwhile, a few generations must manage without breathing at all."
Though, as thus presented, the belief in a transition looks laughable; and
though such derivation of terrestrial vertebrates by direct modification of
piscine vertebrates, is untenable; yet we must not conclude that no
migrations of the kind alleged can have taken place. The adage that "truth
is stranger than fiction," applies quite as much to Nature in general as to
human life. Besides the fact that certain fish actually do "take a walk"
without any obvious reason; and besides the fact that sundry kinds of fish
ramble about on land when prompted by the drying-up of the waters they
inhabit; there is the still more astounding fact that one kind of fish
climbs trees. Few things seem more manifestly impossible, than that a
water-breathing creature without efficient limbs, should ascend eight or
ten feet up the trunk of a palm; and yet the _Anabas scandens_ does as
much. To previous testimonies on this point Capt. Mitchell has recently
added others. Such remarkable cases of temporary changes of media, will
prepare us for conceiving how, under special conditions, permanent changes
of media may have taken place; and for considering how the doctrine of
evolution is elucidated by them.
Inhabitants of the sea, of rivers, and of lakes, are many of them left from
time to time partially or completely without water; and those which show
the power to change their media temporarily or permanently, are in very
many cases of the kinds most liable to be thus deserted by their medium.
Let us consider what the sea-shore shows us. Twice a day the rise and the
fall of the tide covers and uncovers plants and animals, fixed and moving;
and through the alternation of spring and neap tides, it results that the
exposure of the organisms living low down on the beach, varies both in
frequency and duration: while some of them are left dry only once a
fortnight for a very short time, others, a little higher up, are left dry
during two or three hours at several ebb tides every fortnight. Then by
small gradations we come to such as, living at the top of the beach, are
bathed by salt-water only at long intervals; and still higher to some which
are but occasionally splashed in stormy weather. What, now, do we find
among the organisms thus subject to various regular and irregular
alterations of media? Besides many plants and many fixed animals, we find
moving animals of numerous kinds; some of which are confined to the lower
zones of this littoral region, but others of which wander over the whole of
it. Omitting the humbler types, it will suffice to observe that each of the
two great sub-kingdoms, _Mollusca_ and _Arthropoda_, supplies examples of
creatures having a wide excursiveness within this region. We have
gasteropods which, when the tide is down, habitually creep snail-like over
sand and sea-weed, even up as far as high-water mark. We have several kinds
of crustaceans, of which the crab is the most conspicuous, running about on
the wet beach, and sometimes rambling beyond the reach of the water. And
then note the striking fact that each of the forms thus habituated to
changes of media, is allied to forms which are mainly or wholly
terrestrial. On the West Coast of Ireland marine gasteropods are found on
the rocks three hundred feet above the sea, where they are only at long
intervals wetted by the spray; and though between gasteropods of this class
and land-gasteropods the differences are considerable, yet the
land-gasteropods are more closely allied to them than to any other
_Mollusca_. Similarly, the two highest orders of crustaceans have their
species which live occasionally, or almost entirely, out of the water:
there is a kind of lobster in the Mauritius which climbs trees; and there
is the land-crab of the West Indies, which deserts the sea when it reaches
maturity and re-visits it only to spawn. Seeing, thus, how there are many
kinds of marine creatures whose habitats expose them to frequent changes of
media; how some of the higher kinds so circumstanced, show a considerable
adaptation to both media; and how these amphibious kinds are allied to
kinds that are mainly or wholly terrestrial; we shall see that the
migrations from one medium to another, which evolution pre-supposes, are by
no means impracticable. With such evidence before us, the assumption that
the distribution of the _Vertebrata_ through media so different as air and
water, may have been gradually effected in some analogous manner, would not
be altogether unwarranted even had we no clue to the process. We shall
find, however, a tolerably distinct clue. Though rivers, and lakes, and
pools, have no sensible tidal variations, they have their rises and falls,
regular and irregular, moderate and extreme. Especially in tropical
climates, we see them annually full for a certain number of months, and
then dwindling away and drying up. The drying up may reach various degrees
and last for various periods. It may go to the extent only of producing a
liquid mud, or it may reduce the mud to a hardened, fissured solid. It may
last for a few days or for months. That is to say, aquatic forms which are
in one place annually subject to a slight want of water for a short time,
are elsewhere subject to greater wants for longer times: we have gradations
of transition, analogous to those which the tides furnish. Now it is well
known that creatures inhabiting such waters have, in various degrees,
powers of meeting these contingencies. The contained fish either bury
themselves in the mud when the dry season comes, or ramble in search of
other waters. This is proved by evidence from India, Guiana, Siam, Ceylon;
and some of these fish, as the _Anabas scandens_, are known to survive for
days out of the water. But the facts of greatest significance are furnished
by an allied class of _Vertebrata_, almost peculiar to habitats of this
kind. The _Amphibia_ are not, like fish, usually found in waters that are
never partially or wholly dried up; but they nearly all inhabit waters
which, at certain seasons, evaporate, in great measure or
completely--waters in which most kinds of fish cannot exist. And what are
the leading structural traits of these _Amphibia_? They have two
respiratory systems--pulmonic and branchial--variously developed in
different orders; and they have two or four limbs, also variously
developed. Further, the class _Amphibia_ consists of two groups, in one of
which this duality of the respiratory system is permanent, and the
development of the limbs always incomplete; and in the other of which the
branchiæ disappear as the lungs and limbs become fully developed. The
lowest group, the _Perennibranchiata_, have internal organs for aerating
the blood which approach in various degrees to lungs, until "in the
_Siren_, the pulmonic respiration is more extensive and important than the
branchial;" and to these creatures, having a habitat partially aërial and
partially aquatic, there are at the same time supplied, in the shallow
water covering soft mud, the mechanical conditions which render swimming
difficult and rudimentary limbs useful. In the higher group, the
_Caducibranchiata_, we find still more suggestive transformations. Having
at first a structure resembling that which is permanent in the
perennibranchiate amphibian, the larva of the caducibranchiate amphibian
pursues for a time a similar life; but, eventually, while the branchial
appendages dwindle the lungs grow: the respiration of air, originally
supplementary to the respiration of water, predominates over it more and
more, till it replaces it entirely; and an additional pair of legs is
produced. This having been done, the creature either becomes, like the
_Triton_, one which quits the water only occasionally; or, like the Frog,
one which pursues a life mainly terrestrial, and returns to the water now
and then. Finally, if we ask under what conditions this metamorphosis of a
water-breather into an air-breather completes itself, the answer is--it
completes itself at the time when the shallow pools inhabited by the larvæ
are being dried up, or in danger of being dried up, by the summer's
sun.[51]
See, then, how significant are the facts when thus brought together. There
are particular habitats in which animals are subject to changes of media.
In such habitats exist animals having, in various degrees, the power to
live in both media, consequent on various phases of transitional
organization. Near akin to these animals there are some that, after passing
their early lives in the water, acquire more completely the structures
fitting them to live on land, to which they then migrate. Lastly, we have
closely-allied creatures, like the Surinam toad and the terrestrial
salamander, which, though they belong by their structures to the class
_Amphibia_, are not amphibious in their habits--creatures the larvæ of
which do not pass their early lives in the water, and yet go through these
same metamorphoses! Must we then think, like Von Baer, that the
distribution of kindred organisms through different media presents an
insurmountable difficulty? On the contrary, with facts like these before
us, the evolution-hypothesis supplies possible interpretations of many
phenomena that are else unaccountable. After seeing the ways in which such
changes of media are in some cases gradually imposed by physical
conditions, and in other cases voluntarily commenced and slowly increased
in the search after food; we shall begin to understand how, in the course
of evolution, there have arisen strange obscurations of one type by the
externals of another type. When we see land-birds occasionally feeding by
the water-side, and then learn that one of them, the water-ouzel, an
"anomalous member of the strictly terrestrial thrush family, wholly
subsists by diving--grasping the stones with its feet and using its wings
under water"--we are enabled to comprehend how, under pressure of
population, aquatic habits may be acquired by creatures organized for
aërial life; and how there may eventually arise an ornithic type in which
the traits of the bird are very much disguised. On finding among mammals
some that, in search of prey or shelter, have taken to the water in various
degrees, we shall cease to be perplexed on discovering the mammalian
structure hidden under a fish-like form, as it is in the _Cetacea_ and the
_Sirenia_: especially on finding that in the sea-lion and the seals there
are transitional forms. Grant that there has ever been going on that
re-distribution of organisms which we see still resulting from their
intrusions on one another's areas, media, and modes of life; and we have an
explanation of those multitudinous cases in which homologies of structure
are complicated with analogies. And while it accounts for the occurrence in
one medium of organic types fundamentally organized for another medium, the
doctrine of evolution accounts also for the accompanying unfitnesses.
Either the seal has descended from some mammal which little by little
became aquatic in its habits, in which case the structure of its hind limbs
has a meaning; or else it was specially framed for its present habitat, in
which case the structure of its hind limbs is incomprehensible.
§ 140. The facts respecting distribution in Time, which have more than any
others been cited both in proof and in disproof of evolution, are too
fragmentary to be conclusive either way. Were the geological record
complete, or did it, as both Uniformitarians and Progressionists have
commonly assumed, give us traces of the earliest organic forms; the
evidence hence derived, for or against, would have had more weight than any
other evidence. As it is, all we can do is to see whether such fragmentary
evidence as remains, is congruous with the hypothesis.
Palæontology has shown that there is a "general relation between lapse of
time and divergence of organic forms" (§ 107); and that "this divergence is
comparatively slow and continuous where there is continuity in the
geological formations, but is sudden and comparatively wide wherever there
occurs a great break in the succession of strata." Now this is obviously
what we should expect. The hypothesis implies structural changes that are
not sudden but gradual. Hence, where conformable strata indicate a
continuous record, we may anticipate successions of forms only slightly
different from one another; while we may rationally look for marked
contrasts between the groups of forms fossilized in adjacent strata, where
there is evidence of a great blank in the record.
The permanent disappearances of species, of genera, and of orders, which we
saw to be a fact tolerably-well established, is also a fact for which the
belief in evolution prepares us. If later organic forms have in all cases
descended from earlier organic forms, and have diverged during their
descent, both from their prototypes and from one another; then it follows
that such of them as become extinct at any epoch, will never re-appear at a
subsequent epoch; since there can never again arise a concurrence and
succession of conditions such as those under which each type was evolved.
Though comparisons of ancient and modern organic forms, prove that many
types have persisted through enormous periods of time, without undergoing
great changes; it was shown that such comparisons do not disprove the
occurrence in other organic forms, of changes great enough to produce what
are called different types. The result of inductive inquiry we saw to be,
that while a few modern higher types yield signs of having been developed
from ancient lower types; and that while there are many modern types which
_may_ have been thus developed, though we are without evidence that they
have been so; yet that "any admissible hypothesis of progressive
modification must be compatible with persistence without progression
through indefinite periods." Now these results are quite congruous with the
hypothesis of evolution. As rationally interpreted, evolution must in all
cases be understood to result, directly or indirectly, from the incidence
of forces. If there are no changes of conditions entailing organic changes,
organic changes are not to be expected. Only in organisms which fall under
conditions leading to additional modifications answering to additional
needs, will there be that increased heterogeneity which characterizes
higher forms. Hence, though the facts of palæontology cannot be held
conclusive proof of evolution, yet they are congruous with it; and some of
them yield it strong support.
§ 141. One general truth respecting distribution in Time, is profoundly
significant. If, instead of contemplating the relations among past forms of
life taken by themselves, we contemplate the relations between them and the
forms now existing, we find a connexion which is in harmony with the belief
in evolution but irreconcilable with any other belief.
Note, first, how full of meaning is the close kinship existing between the
aggregate of organisms now living, and the aggregate of organisms which
lived in the most recent geologic times. In the last-formed strata, nearly
all the imbedded remains are those of species which still flourish. Strata
a little older contain a few fossils of species now extinct, though,
usually, species greatly resembling extant ones. Of the remains found in
strata of still earlier date, the extinct species form a larger percentage;
and the differences between them and the allied species now living are more
marked. That is to say, the gradual change of organic types in Time, which
we before saw is indicated by the geological record, is equally indicated
by the relation between existing organic types and organic types of the
epochs preceding our own. The evidence completely accords with the belief
in a descent of present life from past life. Doubtless such a kinship is
not incongruous with the doctrine of special creations. It may be argued
that the introduction, from time to time, of new species better fitted to
the somewhat changed conditions of the Earth's surface, would result in an
apparent alliance between our living Flora and Fauna, and the Floras and
Faunas that lately lived. No one can deny it. But on passing from the most
general aspect of the alliance to its more special aspects, we shall find
this interpretation completely negatived.
For besides a close kinship between the aggregate of surviving forms and
the aggregate of forms which have died out in recent geologic times; there
is a peculiar connexion of like nature between present and past forms in
each great geographical region. The instructive fact, before cited from Mr.
Darwin, is the "wonderful relationship in the same continent between the
dead and the living." This relationship is not explained by the supposition
that new species have been at intervals supernaturally placed in each
habitat, as the habitat became modified; since, as we saw, species are by
no means uniformly found in the habitats to which they are best adapted. It
cannot be said that the marsupials imbedded in recent Australian strata,
having become extinct because of unfitness to some new external condition,
the existing marsupials were then specially created to fit the modified
environment; since sundry animals found elsewhere are so much more in
harmony with these new Australian conditions that, when taken to Australia,
they rapidly extrude the marsupials. While, therefore, the similarity
between the existing Australian Fauna and the Fauna which immediately
preceded it over the same area, is just that which the belief in evolution
leads us to expect; it is a similarity which cannot be otherwise accounted
for. And so is it with parallel relations in New England, in South America,
and in Europe.
§ 142. Given, then, that pressure which species exercise on one another, in
consequence of the universal overfilling of their respective
habitats--given the resulting tendency to thrust themselves into one
another's areas, and media, and modes of life, along such lines of least
resistance as from time to time are found--given besides the changes in
modes of life, hence arising, those other changes which physical
alterations of habitats necessitate--given the structural modifications
directly or indirectly produced in organisms by modified conditions; and
the facts of distribution in Space and Time are accounted for. That
divergence and re-divergence of organic forms, which we saw to be shadowed
forth by the truths of classification and the truths of embryology, we see
to be also shadowed forth by the truths of distribution. If that aptitude
to multiply, to spread, to separate, and to differentiate, which the human
races have in all times shown, be a tendency common to races in general, as
we have ample reason to assume; then there will result those kinds of
spacial relations and chronological relations among the species, and
genera, and orders, peopling the Earth's surface, which we find exist. The
remarkable identities of type discovered between organisms inhabiting one
medium, and strangely modified organisms inhabiting another medium, are at
the same time rendered comprehensible. And the appearances and
disappearances of species which the geological record shows us, as well as
the connexions between successive groups of species from early eras down to
our own, cease to be inexplicable.