074 APPENDIX B. 



versy this changed structure of the foot, given occasion for 

 strengthening the opposite case. 

 No reply. 



We come now to Professor Weismann's endeavour to disprove 

 my second thesis that it is impossible to explain by natural 

 selection alone the co-adaptation of co-operative parts. It is 

 thirty years since this was set forth in The Principles of Biology. 

 In 166 I instanced the enormous horns of the extinct Irish elk, 

 and contended that in this, and in kindred cases, where for the 

 efficient use of some one enlarged part many other parts have to 

 be simultaneously enlarged, it is out of the question to suppose 

 that they can have all spontaneously varied in the required pro- 

 portions. In " The Factors of Organic Evolution," by way of 

 enforcing this argument, which had, so far as I know, never been 

 met, I dwelt upon the aberrant structure of the giraffe. And 

 then, in the essay which initiated this controversy, I brought 

 forward yet a third case that of an animal which, previously 

 accustomed only to walking, acquires the power of leaping. 



In the first of his articles in the Contemporary Review (Sep- 

 tember, 1893), Professor Weismann made no direct reply, but he 

 made an indirect reply. He did not attempt to show how there 

 could have taken place in the stag the " harmonious variation of 

 the different parts that co-operate to produce one physiological 

 result" (p. 311) ; but he contended that such harmonious varia- 

 tion must have taken place, because the like has taken place in 

 "the neuters of state-forming insects" ''animal forms which do 

 not reproduce themselves, but are always propagated anew by 

 parents which are unlike them" (p. 313), and which therefore 

 cannot have transmitted acquired characters. Singling out those 

 soldier-neuters which exist among certain kinds of ants, he de- 

 scribed (p. 318) the many co-ordinated parts required to make 

 their fighting organs efficient. He then argued that the required 

 simultaneous changes can " only have arisen by a selection of the 

 parent-ants dependent on the fact that those parents which pro- 

 duced the best workers had always the best prospect of the per- 

 sistence of their colony. No other explanation is conceivable; and 

 it is just because no other explanation is conceivable, that it is neces- 

 sary for UK to accept the principle of natural selection" (pp. 318-9). 



[This passage initiated a collateral controversy, which, as con- 

 tinually happens, has greatly obscured the primary controversy. 

 It became a question whether these forms of neuter insects have 

 arisen as Professor Weismann assumes, or whether they have 

 arisen from arrested development consequent upon innutrition. 

 To avoid entanglements I must for the present pass over this col- 



