The same truth is exemplified if we turn to certain great periods of the earth’s history—­as, for example, the Mesozoic epoch.  There are groups of reptiles, such as the Ichthyosauria and the Plesiosauria, which appear shortly after the commencement of this epoch, and they occur in vast numbers.  They disappear with the chalk and, throughout the whole of the great series of Mesozoic rocks, they present no such modifications as can safely be considered evidence of progressive modification.

Facts of this kind are undoubtedly fatal to any form of the doctrine of evolution which postulates the supposition that there is an intrinsic necessity, on the part of animal forms which have once come into existence, to undergo continual modification; and they are as distinctly opposed to any view which involves the belief, that such modification as may occur, must take place, at the same rate, in all the different types of animal or vegetable life.  The facts, as I have placed them before you, obviously directly contradict any form of the hypothesis of evolution which stands in need of these two postulates.

But, one great service that has been rendered by Mr. Darwin to the doctrine of evolution in general is this:  he has shown that there are two chief factors in the process of evolution:  one of them is the tendency to vary, the existence of which in all living forms may be proved by observation; the other is the influence of surrounding conditions upon what I may call the parent form and the variations which are thus evolved from it.  The cause of the production of variations is a matter not at all properly understood at present.  Whether variation depends upon some intricate machinery—­if I may use the phrase—­of the living organism itself, or whether it arises through the influence of conditions upon that form, is not certain, and the question may, for the present, be left open.  But the important point is that, granting the existence of the tendency to the production of variations; then, whether the variations which are produced shall survive and supplant the parent, or whether the parent form shall survive and supplant the variations, is a matter which depends entirely on those conditions which give rise to the struggle for existence.  If the surrounding conditions are such that the parent form is more competent to deal with them and flourish in them, than the derived forms, then, in the struggle for existence, the parent form will maintain itself and the derived forms will be exterminated.  But if, on the contrary, the conditions are such as to be more favourable to a derived than to the parent form, the parent form will be extirpated and the derived form will take its place.  In the first case, there will be no progression, no change of structure, through any imaginable series of ages; in the second place, there will be modification and change of form.