“We thus see, that although there is a clear and great difference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing of distinct species. And we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first crosses are incidental on unknown differences in their reproductive systems.... The facts by no means seem to indicate that the greater or lesser difficulty of either grafting or crossing various species has been a special endowment; although in the case of crossing, the difficulty is as important for the endurance and stability of specific forms, as in the case of grafting it is unimportant for their welfare.”
We cannot do better, in bringing this long criticism of the Darwinian theory to an end, than by considering the way in which Weismann has attempted in his paper on “Germinal Selection” to solve one of the “patent contradictions” of the selection theory. He calls attention, in doing so, to what he regards as a vital weakness of the theory in the form in which it was left by Darwin himself. Weismann says:—
“The basal idea of the essay—the existence of Germinal Selection—was propounded by me some time since,[15] but it is here for the first time fully set forth and tentatively shown to be the necessary complement of the process of selection. Knowing this factor, we remove, it seems to me, the patent contradiction of the assumption that the general fitness of organisms, or the adaptations _necessary_ to their existence, are produced by _accidental_ variations—a contradiction which formed a serious stumbling-block to the theory of selection. Though still assuming that the _primary_ variations are ‘accidental,’ I yet hope to have demonstrated that an interior mechanism exists which compels them to go on increasing in a definite direction, the moment selection intervenes. _Definitely directed variation exists_, but not predestined variation, running on independently of the life conditions of the organism, as Nägeli, to mention the most extreme advocate of this doctrine, has assumed; on the contrary, the variation is such as is elicited and controlled by those conditions themselves, though indirectly.”