There remains to be considered Darwin's own explanation of the admitted deficiency of palæontological evidence.
The main cause [he writes][316] of innumerable intermediate links [between different forms] not now occurring everywhere throughout nature, depends on the very process of natural selection, through which new varieties continually take the places of and supplant their parent-forms. But just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed, be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.
How imperfect this record is he proceeds to argue at length, and he has no difficulty in showing how much of it has at one time or other been defaced by natural causes, and how small a portion has been laid open to our inspection. But although his demonstration on this point is continually quoted, as though it solved the difficulty, it does not appear that it need detain us long.
It is, in the first place, obvious that the absence of evidence cannot prove the truth of the theory of Evolution or any other, and it is proof of that theory which is required. Apart from palæontological facts, as Professor Huxley has told us, there can be no conclusive evidence one way or the other; and if the geological record be not sufficiently complete to supply such evidence, the theory cannot possibly claim to be scientifically established.
Is it not also, as M. de Quatrefages has remarked, very singular that precisely that evidence must be supposed always to have perished which the Evolution theory imperatively requires, while so much remains which appears to contradict it?
But, moreover, as Mr. Carruthers says, incomplete though the record undoubtedly is, and limited as is our knowledge even of what exists,--there still remains a vast mass of information which it has actually supplied, and there seems to be no reason for denying that, as to the particular point under consideration, its testimony is ample. If, as on the principles of genetic Evolution must be the case, there were in each line of descent no successive species or genera, made up of forms clustered round one point in the course of development more than another, how comes it that we find always and everywhere just such isolated clusters, naturally forming genera and species; and that in no single instance do we find any trace of the graduated series linking them together? Is it not quite impossible to suppose, that at all points in Nature we stumble upon exactly those instances which disguise, and apparently contradict, the method upon which she invariably works?