“As the capacity of one plant to be grafted or budded on another is unimportant for their welfare in a state of nature, I presume that no one will suppose that this capacity is a specially endowed quality, but will admit that it is incidental on differences in the laws of growth of the two plants. We can sometimes see the reason why one tree will not take on another, from differences in their rate of growth, in the hardness of their wood, in the period of the flow or nature of their sap, etc.; but in a multitude of cases we can assign no reason whatever. Great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adapted to widely different climates, do not always prevent the two grafting together. As in hybridization, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees belonging to quite distinct families; and, on the other hand, closely allied species, and varieties of the same species, can usually, but not invariably, be grafted with ease. But this capacity, as in hybridization, is by no means absolutely governed by systematic affinity. Although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. The pear can be grafted far more readily on the quince, which is ranked as a distant genus, than on the apple, which is a member of the same genus. Even different varieties of the pear take with different degrees of facility on the quince; so do different varieties of the apricot and peach on certain varieties of the plum.”
“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.”
Weismann’s Germinal Selection
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.”
[15]. Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer, Jena, 1895.
“The real aim of the present essay is to rehabilitate the principle of selection. If I should succeed in reinstating this principle in its emperilled rights, it would be a source of extreme satisfaction to me; for I am so thoroughly convinced of its indispensability as to believe that its demolition would be synonymous with the renunciation of all inquiry concerning the causal relation of vital phenomena. If we could understand the adaptations of nature, whose number is infinite, only upon the assumption of a teleological principle, then, I think, there would be little inducement to trouble ourselves about the causal connection of the stages of ontogenesis, for no good reason would exist for excluding teleological principles from this field. Their introduction, however, is the ruin of science.”[[16]]
[16]. Translated by J. McCormack. The Open Court Publishing Company. The following quotations are also taken from this translation.
Weismann states that those critics who maintain that selection cannot create, but only reject, “fail to see that precisely through this rejection its creative efficacy is asserted.” There is raised here, though not for the first time, a point that is of no small importance for both Darwinians and anti-Darwinians to consider; for, without further examination, it is by no means self-evident, as Weismann implies, that by exterminating all variations that are below the average the standard of successive generations could ever be raised beyond the most extreme fluctuating variation. At least this appears to be the case if individual, fluctuating variations be the sort selected, and it is to this kind of variation to which Weismann presumably refers. Without discussing this point here, let us examine further what Weismann has to say. He thinks that while in each form there may be a very large number of possible variations, yet there are also impossible variations as well, which do not appear. “The cogency, the irresistible cogency as I take it, of the principle of selection is precisely its capacity of explaining why fit structures always arise, and this certainly is the great problem of life.” Weismann points out that it is a remarkable fact that to-day, after science has been in possession of this principle for something over thirty years, “during which time she has busily occupied herself with its scope, the estimation in which the theory is held should be on the decline.” “It would be easy to enumerate a long list of living writers who assign to it a subordinate part only in evolution, or none at all.” “Even Huxley implicitly, yet distinctly, intimated a doubt regarding the principle of selection when he said: ‘Even if the Darwinian hypothesis were swept away, evolution would still stand where it is.’ Therefore he, too, regarded it as not impossible that this hypothesis should disappear from among the great explanatory principles by which we seek to approach nearer to the secrets of nature.”
Weismann is not, however, of this opinion, and believes that the present depression is only transient, because it is only a reaction against a theory that had been exalted to the highest pinnacle. He thinks that the principle of selection is not overestimated, but that naturalists imagined too quickly that they understood its workings. “On the contrary, the deeper they penetrated into its workings the clearer it appeared that something was lacking, that the action of the principle, though upon the whole clear and representable, yet when carefully looked into encountered numerous difficulties, which were formidable, for the reason that we were unsuccessful in tracing out the actual details of the individual process, and, therefore, in fixing the phenomenon as it actually occurred. We can state in no single case how great a variation must be to have selective value, nor how frequently it must occur to acquire stability. We do not know when and whether a desired useful variation really occurs, nor on what its appearance depends; and we have no means of ascertaining the space of time required for the fulfilment of the selective processes of nature, and hence cannot calculate the exact number of such processes that do and can take place at the same time in the same species. Yet all this is necessary if we wish to follow out the precise details of a given case.