Page:The Zoologist, 4th series, vol 2 (1898).djvu/417

Rh altered survived as the "fittest." For, as remarked by Paul in a sense that cannot, however, be called biological, that without he had known the law, neither had he known sin; so, until animal life had developed from its little differentiated phase to the advanced stage when a struggle for existence ensued, natural selection scarcely existed as a controlling force. There was doubtless what may be suggested as an evolutionary impulse, This evolutionary impulse might be perhaps defined in the words of Matthew Arnold as applied to another subject: "That awful and benevolent impulsion of things within us and without us, which we can concur with, indeed, but cannot create." Apparently similar to the "idioplasm" of Nageli. On the other hand, the terms "impulse" and "stimulus" lack a clear definition. "Here, as in so many similar cases, a phrase, a technical term, a word, is introduced to designate the process observed, and not infrequently those who use it ultimately come to think they have given an explanation of the process, while they really have only stated it. This is especially the case with the term ' stimulus.' What is a stimulus? From the present state of our knowledge we cannot yet give a concise answer to this question, consequently explanations in which this word is inserted are, as explanations, incomplete" (Kerner and Oliver, 'Nat. Hist. Plants,' vol. i. pp. 776–7). Mr. Mivart would apparently recognize this internal force as "instinct," postulating: "Instead, then, of explaining instinct by reflex action (as a reflex action accompanied by sensation), I would explain reflex action, processes of repair, and processes of individual and specific evolution, by Instinct—the wonderful action and nature of which we know as it exists in our own personal activity " (Proc. Zool. Soc. 1884, p. 473). Mr. Orr uses several equivalents, such as elementary nervousness," which makes possible and necessary the formation of co-ordinations and associations as the result of repetition of the necessitated reactions." Inherited impulse of growth, "which in combination with external forces constantly drives the organism forward on its course of development, and, even while the environing forces remain the same, is constantly exposing the developing individual to new stimuli, because it is constantly changing the individual." Hereditary impulse, "which is the result of the long previous history of the organism" ('Theory of Development and Heredity,' pp. 108, 143, 198). In all these terms we are reminded of the "internal perfecting tendency" of Aristotle. Again, Kolliker's idea of the evolution of forms from "internal causes" on the basis of a " general law of evolution"; Kolliker subsequently explained that his internal causes were physico-chemical (see Eimer, 'Organic Evolution,' Eng. transl. pp. 49, 50). Mr. Dixon recognizes this factor in the migration of birds: "Young birds are not born with this hereditary know