ASSIMILATIVE COLOURATION. 385 
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,* 
into account, both as actors under nature, and as interpreters of nature” 
(ibid. p. 81). John Stuart Mill has given a similar definition (‘Three Essays 
on Religion,’ p. 6). 
* 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 im- 
pulse, ‘which is the result of the long previous history of the organism” 
(‘Theory of Development and Heredity,’ pp. 108, 148, 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 Himer, ‘Organic Evo- 
lution,’ Eng. transl. pp. 49, 50). Mr. Dixon recognizes this factor in the 
migration of birds: ‘“‘ Young birds are not born with this hereditary know- 
