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Show 146 MR. DARWIN'S WORK AND and its modifications may, occasionally, all find the conditions fitted for their existence; and though they come into competition, to a certain extent, with one another, the derivative ~pecies may not necessarily Extirpate the primitive one, or vice versa. Now palreontology shows us many facts which are perfectly harmonious with these observed effects of the process by which Mr. Darwin supposes species to have originated, but which appear to me to be totally inconsistent with any other hypothesis which has been proposed. There are some groups of animals and plants, in the fossil. world, which have been said to belong to " persistent types," because they have persisted, with very little change indeed, through a very ·great range of time, while everything about them has changed largely. There are families of fishes whose type of construction has persisted all the way from the carboniferous rock right up to the cretaceous; and others which have lasted through almost the whole range of the secondary rocks, and from the lias to the older tertiaries. It is something stupendous this -to consider a genus lasting without essential modifications through all this enormous lapse of time w bile almost everything else was changed and modified. Thus I have no doubt that Mr. Darwin's hypothesis will be found competent to explain the majority of the phenomena exhibited by species in nature; but in an earlier lecture I spoke cautiously with respect to its power of explaining all the physiological peculiarities of species. There is, in fact, one set of these peculiarities which the theory of selective modification, as it stands at present, THE PHENOMENA OF ORGANIC NATURE. 147 is not wholly competent to explain, and that is the group of phenomena which I mentioned to you under the name of Hybridism, and which I explained to consist in the sterility of the offspring of certain species when crossed one with another. It matters not one whit whether this sterility is universal, or whether it exists only in a single case. Every hypothesis is bound to explain, or, at any rate, not be inconsistent with, the whole of the facts which it professes to account for; and if there is a single one of these facts which can be shown to be inconsistent with (I do not merely mean inexplicable by, but contrary to,) the hypothesis, the hypothesis falls to the ground,-it is worth nothing. One fact with which it is positively inconsistent is worth as much, and as powerful in negativing the hypothesis, as five hundred. If I am right in thus defining the obligations of a hypothesis, Mr. Darwin, in order to ·place his views beyond the reach of all possible assault, ought to be able to demonstrate the possibility of developing from a particular stock , by selective breeding, two forms, which should either be unable to cross one with another, or whose cross-bred offspring should be infertile with one another. For, you see, if you have not done that you have not strictly fulfilled all the conditions of the problem; you have not shown that you can produce, by the cause assumed, all the phenomena which you have in nature. Here are the phenomena of Hybridism staring you in the face, and you cannot say, "I can, by selective modification, produce these same results." Now, it is admitted on all hands that, at present, so far as experiments have gone, it has not been found possible to pro- |