D.— ZOOLOGY. 99 



stripe will not be so surprising as it is on the theory of direct environmental 

 influence. 



But there do remain many obscure points : for example, Sumner in 

 his cultures observed the appearance, either as mutations or more probably 

 simply by segregation, of certain colour conditions which were inherited 

 as simple single-factor Mendelian unit characters. These, a recessive Yellow, 

 Albinism, &c., were not seen by him in the large collections of wild mice 

 which he made, and as they occurred in comparatively large numbers in 

 the cage-bred animals they should have done so. Is their absence due 

 to a natural selection ? or, if not, to what is it due ? Thus we come back 

 to the question of the existence of adaptation in the sjDecial form which 

 is demanded by the Darwinian theory. And for such close and detailed 

 correlation of structure with conditions of life we have as yet no evidence 

 though much assumption. 



There remains one type of adaptation which is perhaps of greater 

 importance than those which we have been considering. 



Perhaps the most striking of all the phenomena of life is the power 

 which all animals and plants possess of so regulating their functioning, and 

 when necessary their morphology, that their life is continued in equilibrium 

 with the conditions under which they find themselves. 



This adaptation is familiar in the automatic regulation of the action 

 of the heart of a mammal and of its respiration to increased or decreased 

 activity, and in the numberless similar adjustments of physiological 

 processes. 



Mr. Pantin tells me that his own experience has shown him that the 

 physiological condition of marine animals is different in winter and 

 summer, although I believe it has not yet been shown that this variation 

 has an adaptive significance. 



How far this adaptation to internal conditions is brought about by 

 the same mechanism as adaptation to the environment I do not know. 

 In those cases where the body fluid is nothing but sea water, as in 

 echinoderms, it does seem evident that to a considerable extent internal 

 and external environments are one. 



But how far such physiological adaptations are of the same nature as 

 those internal morphological adaptations which alter the relative sizes of 

 parts in ways determined by geometrical considerations of squares and 

 cubes, and produce analogous modifications in other structural features, 

 there is no evidence. What is certain, however, is that these, which are 

 the fundamental things in evolution, lie open to experiment. 



Thus the present position of zoology is unsatisfactory ; we know as 

 surely as we ever shall that evolution has occurred. But we do not know 

 how this evolution has been brought about. The data which we have 

 accumulated are inadequate, not in quantity but in their character to 

 allow us to determine which, if any, of the proposed explanations is a 

 vera causa. 



But it appears that the experimental method rightly used will in the 

 end give us, if not the solution of our problem, at least the power of 

 analysing it and isolating the various factors which enter into it. 



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