Darwin, and After Darwin, Volumes 1 and 3 / An Exposition of the Darwinian Theory and a Discussion of Post-Darwinian Questions

by George John Romanes

That such estimates of the destructive powers of nature are not mere matters of speculative reasoning may be amply shown by stating one single fact, which, like so many others where the present subject is concerned, we owe to the generalizations of Darwin. Plutonic rocks, being those which have emerged from subterranean heat of melting intensity, must clearly at some time or another have lain beneath the whole thickness of sedimentary deposits, which at that time occupied any part of the earth’s surface where we now find the Plutonic rocks exposed to view. Or, in other words, wherever we now find Plutonic rocks at the surface of the earth, we must conclude that all the sedimentary rocks by which they were covered when in a molten state have since been entirely destroyed; several vertical miles of the only kinds of rocks in which fossils can possibly occur must in all such cases have been abolished in toto. Now, in many parts of the world metamorphic rocks—which have thus gradually risen from Plutonic depths, while miles of various other rock-formations have been removed from their now exposed surfaces—cover immense areas, and therefore testify by their present horizontal range, no less than by their previously vertical depth, to the enormous scale on which a total destruction has taken place of everything that once lay above them. For instance, the granitic region of Parime is at least nineteen times the size of Switzerland; a similar region south of the Amazon is probably larger than France, Spain, Italy, and Great Britain all put together; and, more remarkable still, over the area of the United States and Canada, granitic rocks exceed in the proportion of 19 to 12½ the whole of the newer Palæozoic formations. Lastly, after giving these examples, Darwin adds the important consideration, that “in many regions the metamorphic and granitic rocks would be found much more widely extended than they appear to be, if all the sedimentary beds were removed which rest unconformably on them, and which could not have formed part of the original mantle under which they were crystallized.”

The above is a brief condensation of the already condensed statement which Darwin has given of the imperfection of the geological record; but I think it is enough to show, in a general way, how precarious must be the nature of any objections to the theory of evolution which are founded merely upon the silence of palæontology in cases where, if the record were anything like complete, we should be entitled to expect from it some positive information. But, as we have seen in the text, imperfect though the record be, in as far as it furnishes positive information at all, this is well-nigh uniformly in favour of the theory; and therefore, even on grounds of palæontology alone, it appears to me that Darwin is much too liberal where he concludes his discussion by saying,—"Those who believe that the geological record is in any degree perfect, will undoubtedly at once reject the theory.” If in any measure reasonable, such persons ought rather to examine their title to such a belief; and even if they disregard the consensus of testimony which is yielded by all the biological sciences to the theory of evolution, they ought at least to hold their judgment in suspense until they shall have not only set against the apparently negative testimony which is yielded by geology its unquestionably positive testimony, but also well considered the causes which may—or rather must—have so gravely impaired the geological record.

However, be this as it may, I will now pass on to consider the difficulties and objections which have been brought against the theory on grounds of palæontology.

These may be classified under four heads. First, the absence of varietal links between allied species; second, the sudden appearance of whole groups of species—not only as genera and families, but even sometimes as orders and classes—without any forms leading up to them; third, the occurrence of highly organized types at much lower levels of geological strata than an evolutionist would antecedently expect; and, fourth, the absence of fossils of any kind lower down than the Cambrian strata.

Now all these objections depend on estimates of the imperfection of the geological record much lower than that which is formed by Darwin. Therefore I have arranged the objections in their order of difficulty in this respect, or in the order that requires successively increasing estimates of the imperfection of the record, if they are to be successively answered.

I think that the first of them has been already answered in the text, by showing that even a very moderate estimate of the imperfection of the record is enough to explain why intermediate varieties, connecting allied species, are but comparatively seldom met with. Moreover it was shown that in some cases, where shells are concerned, remarkably well-connected series of such varieties have been met with. And the same applies to species and genera in certain other cases, as in the equine family.

But no doubt a greater difficulty arises where whole groups of species and genera, or even families and orders, appear to arise suddenly, without anything leading up to them. Even this the second difficulty, however, admits of being fully met, when we remember that in very many cases it has been proved, quite apart from the theory of descent, that superjacent formations have been separated from one another by wide intervals of time. And even although it often happens that intermediate deposits which are absent in one part of the world are present in another, we have no right to assume that such is always the case. Besides, even if it were, we should have no right further to assume that the faunas of widely separated geographical areas were identical during the time represented by the intermediate formation. Yet, unless they were identical, we should not expect the fossils of the intermediate formation, where extant, to yield evidence of what the fossils would have been in this same formation elsewhere, had it not been there destroyed. Now, as a matter of fact, “geological formations of each region are almost invariably intermittent"; and although in many cases a more or less continuous record of past forms of life can be obtained by comparing the fossils of one region and formation with those of another region and adjacent formations, it is evident (from what we know of the present geographical distribution of plants and animals) that not a few cases there must have been where the interruption of the record in one region cannot be made good by thus interpolating the fossils of another region. And we must remember it is by selecting the cases where this cannot be done that the objection before us is made to appear formidable. In other words, unless whole groups of new species which are unknown in formation A appear suddenly in formation C of one region (X), where the intermediate formation B is absent; and unless in some other region (Y), where B is present, the fossiliferous contents of B fail to supply the fossil ancestry of the new species in A (X); unless such a state of matters is found to obtain, the objection before us has nothing to say. But at best this is negative evidence; and, in order to consider it fairly, we ought to set against it the cases where an interposition of fossils found in B (Y) does furnish the fossil ancestry of what would otherwise have been an abrupt appearance of whole groups of new species in A (X). Now such cases are neither few nor unimportant, and therefore they deprive the objection of the force it would have had if the selected cases to the contrary were the general rule.

In addition to these considerations, the following, some of which are of a more special kind, appear to me so important that I will quote them almost in extenso.

We continually forget how large the world is, compared with the area over which our geological formations have been carefully examined: we forget that groups of species may elsewhere have long existed, and have slowly multiplied, before they invaded the ancient archipelagoes of Europe and the United States. We do not make due allowance for the intervals of time which have elapsed between our consecutive formations,—longer perhaps in many cases than the time required for the accumulation of each formation. These intervals will have given time for the multiplication of species from some one parent form; and, in the succeeding formation, such groups of species will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might require a long succession of ages, to adapt an organism to some new and peculiar line of life, for instance, to fly through the air; and consequently that the transitional form would often long remain confined to some one region; but that, when this adaptation had once been effected, and a few species had thus acquired a great advantage over other organisms, a comparatively short time would be necessary to produce many divergent forms, which would spread rapidly and widely throughout the world....