240 LAMARCK AND DARWIN 



Darwin's conception of correlation was singularly in- 

 complete. As examples of correlation he advanced such 

 trivial cases as the relation between albinism, deafness and 

 blue eyes in cats, or between the tortoise-shell colour and the 

 female sex. He used the word only in connection with what 

 he called "correlated variation," meaning by this expression 

 " that the whole organisation is so tied together during its 

 growth and development, that when slight variations in any 

 one part occur, and are accumulated through natural selection, 

 other parts become modified" (6th ed., p. 1/7). He took it 

 for granted that the " correlated variations " would be 

 adapted to the original variation which was acted upon by 

 natural selection, and he saw no difficulty in the gradual 

 evolution of a complicated organ like the eye if only the 

 steps were small enough. " It has been objected," he writes, 

 " that in order to modify the eye and still preserve it as a 

 perfect instrument, many changes would have to be effected 

 simultaneously, which, it is assumed, could not be done 

 through natural selection ; but as I have attempted to show 

 in my work on the variation of domestic animals, it is not 

 necessary to suppose that the modifications were all 

 simultaneous, if they were extremely slight and gradual " 

 (6th ed., p. 226). 



In post-Darwinian speculation the difficulty of explaining 

 correlated variation by natural selection alone became more 

 acutely realised, and it was chiefly this difficulty that led 

 Weismann to formulate his hypothesis of germinal selection 

 as a necessary supplement to the general selection theory. 



The change in the conception of correlation which 

 Darwin's influence brought about has been very clearly 

 stated by E. von Hartmann, 1 from whom the following is 

 taken: "While the correlation of parts in the organism was 

 before Darwin regarded exclusively from the standpoint of 

 morphological systematics, Darwin tried to look at it from 

 the standpoint of physiological and genealogical develop- 

 ment, and in so doing he put the standpoint of morphological 

 systematics in the shade. But the more we are now beginning 

 to realise that systematic relationship does not necessarily 



1 Das Problem des Lebens. Biologischc Studicn. Bad Sacha, 1906. 

 See also E. Radl, JUol. Ccntralblatt, xxi., 1901. 



