Volume I Part 19 (1/2)
3. Similarly, and more generally, the Dicotyledonous plants, which first appear in the Cretaceous rocks, appear there suddenly, without any forms leading up to them--notwithstanding that ”we know very well the extensive flora of the underlying Wealden.” Moreover, we have all the three great divisions of the Dicotyledons appearing together, and so highly differentiated that all the species are referred to existing genera, with the exception of a very few imperfectly preserved, and therefore uncertain fragments.
Such being the facts, we may begin by noticing that, even at first sight, they present different degrees of difficulty. Thus, I cannot see that there is much difficulty with regard to those in cla.s.s 2. Only if we were to take the popular (and very erroneous) view of organic evolution as a process which is always and everywhere bound to promote the specialization of organic types--only then ought we to see any real difficulty in the absence of generalized types preceding these existing types. Of course we may wonder why still lower down in the geological series we do not meet with more generalized (or ancestral) types; but this is the difficulty number 3, which we now proceed to examine.
Concerning the other two difficulties, then, the only possible way of meeting that as to the absence of any parent forms lower down in the geological series is by falling back--as in the a.n.a.logous case of animals--upon the imperfection of the geological record. Although it is certainly remarkable that we should not encounter any forms serving to connect the Dicotyledonous plants of the Chalk with the lower forms of the underlying Wealden, we must again remember that difficulties thus depending on the absence of any corroborative record, are by no means equivalent to what would have arisen in the presence of an adverse record--such, for instance, as would have been exhibited had the floras of the Wealden and the Chalk been inverted. But, as the case actually stands, the mere fact that Dicotyledonous plants, where they first occur, are found to have been already differentiated into their three main divisions, is in itself sufficient evidence, on the general theory of evolution, that there must be a break in the record as. .h.i.therto known between the Wealden and the Chalk. Nor is it easy to see how the opponents of this theory can prove their negative by furnis.h.i.+ng evidence to the contrary. And although such might justly be deemed an unfair way of putting the matter, were this the only case where the geological record is in evidence, it is not so when we remember that there are numberless other cases where the geological record does testify to connecting links in a most satisfactory manner. For in view of this consideration the burden of proof is thrown upon those who point to particular cases where there is thus a conspicuous absence of transitional forms--the burden, namely, of proving that such cases are not due merely to a break in the record. Besides, the break in the record as regards this particular case may be apparent rather than real.
For I suppose there is no greater authority on the pure geology of the subject than Sir Charles Lyell, and this is what he says of the particular case in question. ”If the pa.s.sage seem at present to be somewhat sudden from the flora of the Lower or Neocomian to that of the Upper Cretaceous period, the abruptness of the change will probably disappear when we are better acquainted with the fossil vegetation of the uppermost tracts of the Neocomian and that of the lowest strata of the Gault, or true Cretaceous series[59].”
[59] _Elements of Geology_, p. 280.
Lastly, the fact of the flora of the glacial epoch not having exhibited any modifications during the long residence of some of its specific types in Great Britain and elsewhere, is a fact of some importance to the general theory of evolution, since it shows a higher degree of stability on the part of these specific types than might perhaps have been expected, supposing the theory to be true. But I do not see that this const.i.tutes a difficulty against the theory, when we have so many other cases of proved trans.m.u.tation to set against it. For instance, not to go further afield than this very glacial flora itself, it will be remembered that in an earlier chapter I selected it as furnis.h.i.+ng specially cogent proof of the trans.m.u.tation of species. What, then, is the explanation of so extraordinary a difference between Mr. Carruthers'
views and my own upon this point? I believe the explanation to be that he does not take a sufficiently wide survey of the facts.
To begin with, it seems to me that he exaggerates the vicissitudes to which the species of plants that he calls into evidence have been exposed while advancing before, and retreating after, the ice. Rather do I agree with Darwin that ”they would not have been exposed during their long migrations to any great diversity of temperature; and as they all migrated in a body together, their mutual relations will not have been much disturbed; hence, in accordance with the principles indicated in this volume, these forms will not have been liable to much modification[60].” But, be this matter of opinion as it may, a much better test is afforded by those numerous cases all the world over, where arctic species have been left stranded on alpine areas by the retreat of glaciation; because here there is no room for differences of opinion as to a ”change of environment” having taken place. Not to speak of climatic differences between arctic and alpine stations, consider merely the changes which must have taken place in the relations of the thus isolated species to each other, as well as to those of all the foreign plants, insects, &c., with which they have long been thrown into close a.s.sociation. If in _such_ cases no variation or trans.m.u.tation had taken place since the glacial epoch, then indeed there would have been a difficulty of some magnitude. But, by parity of reasoning, whatever degree of difficulty would have been thus presented is not merely discharged, but converted into at least an equal degree of corroboration, when it is found that under such circ.u.mstances, in whatever part of the world they have occurred, some considerable amount of variation and trans.m.u.tation has always taken place,--and this in the animals as well as in the plants. For instance, again to quote Darwin, ”If we compare the present Alpine plants and animals of the several great European mountain-ranges one with another, though many of the species remain identically the same, some exist as varieties, some as doubtful forms or sub-species, and some as distinct yet closely allied species representing each other on the several ranges[61].” Lastly, if instead of considering the case of alpine floras, we take the much larger case of the Old and New World as a whole, we meet with much larger proofs of the same general facts. For, ”during the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they will have been completely cut off from each other.
This separation, as far as the more temperate productions are concerned, must have taken place long ages ago. As the plants and animals migrated southward, they will have become mingled in one great region with the native American productions, and would have had to compete with them; and, in the other great region, with those of the Old World.
Consequently we have here everything favourable for much modification,--for far more modification than with the Alpine productions left isolated, within a much more recent period, on the several mountain ranges and on the arctic lands of Europe and N.
America. Hence it has come, that when we compare the now living productions of the temperate regions of the New and Old Worlds, we find very few identical species; but we find in every cla.s.s many forms, which some naturalists rank as geographical races, and others as distinct species; and a host of closely allied or representative forms which are ranked by all naturalists as specifically distinct[62].”
[60] _Origin of Species_, p. 332.
[61] _Origin of Species_, p. 332.
[62] _Ibid_. pp. 333-4.
In view then of all the above considerations--and especially those quoted from Darwin--it appears to me that far from raising any difficulty against the theory of evolution, the facts adduced by Mr.
Carruthers make in favour of it. For when once these facts are taken in connection with the others above mentioned, they serve to complete the correspondence between degrees of modification with degrees of time on the one hand, and with degrees of evolution, of change of environment, &c., on the other. Or, in the words of Le Conte, when dealing with this very subject, ”It is impossible to conceive a more beautiful ill.u.s.tration of the principles we have been trying to enforce[63].”
[63] _Evolution and its Relation to Religious Thought_, p. 194.
NOTE A TO PAGE 257.
The pa.s.sages in Dr. Whewell's writings, to which allusion is here made, are somewhat too long to be quoted in the text. But as I think they deserved to be given, I will here reprint a letter which I wrote to _Nature_ in March, 1888.
In his essay on the _Reception of the Origin of Species_, Prof.
Huxley writes:--
”It is interesting to observe that the possibility of a fifth alternative, in addition to the four he has stated, has not dawned upon Dr. Whewell's mind” (_Life and Lectures of Charles Darwin_, vol. ii, p. 195).
And again, in the article _Science_, supplied to _The Reign of Queen Victoria_, he says:--
”Whewell had not the slightest suspicion of Darwin's main theorem, even as a logical possibility” (p 365).
Now, although it is true that no indication of such a logical possibility is to be met with in the _History of the Inductive Sciences_, there are several pa.s.sages in the _Bridgewater Treatise_ which show a glimmering idea of such a possibility. Of these the following are, perhaps, worth quoting. Speaking of the adaptation of the period of flowering to the length of a year, he says:--
”Now such an adjustment must surely be accepted as a proof of design, exercised in the formation of the world. Why should the solar year be so long and no longer? or, this being such a length, why should the vegetable cycle be exactly of the same length? Can this be chance?... And, if not by chance, how otherwise could such a coincidence occur than by an intentional adjustment of these two things to one another; by a selection of such an organization in plants as would fit them to the earth on which they were to grow; by an adaptation of construction to conditions; of the scale of construction to the scale of conditions? It cannot be accepted as an explanation of this fact in the economy of plants, that it is necessary to their existence; that no plants could possibly have subsisted, and come down to us, except those which were thus suited to their place on the earth. This is true; but it does not at all remove the necessity of recurring to design as the origin of the construction by which the existence and continuance of plants is made possible. A watch could not go unless there were the most exact adjustment in the forms and positions of its wheels; yet no one would accept it as an explanation of the origin of such forms and positions that the watch would not go if these were other than they were. If the objector were to suppose that plants were originally fitted to years of various lengths, and that such only have survived to the present time as had a cycle of a length equal to our present year, or one which could be accommodated to it, we should reply that the a.s.sumption is too gratuitous and extravagant to require much consideration.”