Introduction. 19 



mechanisms in nature ? " — he writes, " Self-adaptation 

 does not profess to be a vera causa at all : for the 

 true causes of variation can only be found in the 

 answer to your [above] questions, and I must say 

 at once, these questions cannot be answered." That 

 is, they cannot be answered on the hypothesis of 

 self-adaptation, which is therefore a statement of 

 the facts of adaptation as distinguished from an 

 explanation of them. Nevertheless, two things have 

 here to be noted. In the first place, the statement 

 of facts which Mr. Henslow has collected is of con- 

 siderable theoretical importance as tending to show 

 that there are probably causes of an internal kind 

 (i.e. other than natural selection) which have been 

 largely concerned in the adaptive n^odification of 

 plants. And, in the second place, it is not quite true 

 that the theory of self-adaptation is. as its author 

 says in the sentences above quoted, a mere statement 

 of the facts of adaptation, without any attempt at 

 explaining their causes. For in his published words 

 he does attempt to do so^ And, although I think 

 his attempt is a conspicuous failure, I ought in fair- 

 ness to give examples of it. His books are almost 

 exclusively concerned in an application of his theory 

 to the mechanisms of flowers for securing their own 

 fertilization. These mechanisms he ascribes, in the 

 case of entomophylous flowers, to the "thrusts," 

 "strains," and other "irritations" supplied to the 

 flowers by their insect visitors, and consequent "reac- 

 tions " of the vegetable "protoplasm." But no 

 attempt is made to show why these " reactions " 



' Floral structures (Internat. Sc. Ser. Ixiv. 1888): The Making of 

 Flowers (Romance of Science Ser. 1891) ; and Linn. Soc. Papers 1893-4. 



C 2 



