Darwin’s view is that, during the vast ages of time now under consideration, it is probable that the distribution of sea and land over the earth’s surface has not been uniformly the same, even as regards oceans and continents. Now, if this were the case, “it might well happen that strata which had subsided some miles nearer to the centre of the earth, and which had been pressed on by an enormous weight of superincumbent water, might have undergone far more metamorphic action than strata which have always remained nearer to the surface. The immense areas in some parts of the world, for instance in South America, of naked metamorphic rocks, which must have been heated under great pressure, have always seemed to me to require some special explanation; and we may perhaps believe that we see, in these large areas, the many formations long anterior to the Cambrian epoch in a completely metamorphosed and denuded condition[55].” The probability of this view he sustains by certain general considerations, as well as particular facts touching the geology of oceanic islands, &c.
On the whole, then, it seems to me but reasonable to conclude, with regard to all four objections in question, as Darwin concludes with regard to them:—
For my part, following out Lyell’s metaphor, I look at the geological record as a history of the world imperfectly kept, written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to us to have been abruptly introduced. On this view, the difficulties above discussed are greatly diminished, or even disappear[56].
As far as I can see, the only reasonable exception that can be taken to this general view of the whole matter, is one which has been taken from the side of astronomical physics.
Put briefly, it is alleged by one of the highest authorities in this branch of science, that there cannot have been any such enormous reaches of unrecorded time as would be implied by the supposition of there having been a lost history of organic evolution before the Cambrian period. The grounds of this allegation I am not qualified to examine; but in a general way I agree with Prof. Huxley in feeling that, from the very nature of the case, they are necessarily precarious,—and this in so high a degree that any conclusions raised on such premises are not entitled to be deemed formidable[57].
Turning now to plants, the principal and the ablest opponent of the theory of evolution is here unquestionably Mr. Carruthers[58]. The difficulties which he adduces may be classified under three heads, as follows:—
1. There is no evidence of change in specific forms of existing plants. Not only are the numerous species of plants which have been found in Egyptian mummies indistinguishable from their successors of to-day; but, what is of far more importance, a large number of our own indigenous plants grew in Great Britain during the glacial period (including under this term the warm periods between those of successive glaciations), and in no one case does it appear that any modification of specific type has occurred. This fact is particularly remarkable as regards leaves, because on the one hand they are the organs of plants which are most prone to vary, while on the other hand they are likewise the organs which lend themselves most perfectly to the process of fossilization, so that all details of their structure can be minutely observed in the fossil state. Yet the interval since the glacial period, although not a long one geologically speaking, is certainly what may be called an appreciable portion of time in the history of Dicotyledonous plants since their first appearance in the Cretaceous epoch. Again, if we extend this kind of enquiry so as to include the world as a whole, a number of other species of plants dating from the glacial epoch are found to tell the same story—notwithstanding that, in the opinion of Mr. Carruthers, they must all have undergone many changes of environment while advancing before, and retreating after, successive glaciations in different parts of the globe. Or, to quote his own words:—"The various physical conditions which of necessity affected these {41} species in their diffusion over such large areas of the earth’s surface in the course of, say, 250,000 years, should have led to the production of many varieties; but the uniform testimony of the remains of this considerable pre-glacial flora, as far as the materials admit of a comparison, is that no appreciable change has taken place.”
2. There is no appearance of generalized forms among the earliest plants with which we are acquainted. For example, in the first dry land flora—the Devonian—we have representatives of the Filices, Equisetaceæ, and Lycopodiaceæ, all as highly specialized as their living representatives, and exhibiting the differential characters of these closely related groups. Moreover, these plants were even more highly organized than their existing descendants in regard to their vegetative structure, and in some cases also in regard to their reproductive organs. So likewise the Gymnosperms of that time show in their fossil state the same highly organized woody structure as their living representatives.
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.