NATURAL SELECTION. 393 



however, impossible ; yet natural selection has un- 

 doubtedly been the cause of their survival. Professor 

 Poulton has demonstrated (p. 230) that the protective 

 colors in lepidopterous pupae are produced directly 

 by the influences of light on the nerves of the ani- 

 mal and its reflex action on the pigment depository 

 process. 



The first objection to the belief that natural selec- 

 tion is the primary cause of organic evolution has 

 been already stated as follows : "A selection cannot 

 be the cause of those alternatives from which it se- 

 lects. The alternatives must be presented before the 

 selection can commence." But the supporters of the 

 view that natural selection is the origin of variation 

 allege that it produces this result by the continued 

 survival of minute differences which are useful, thus 

 accumulating variation. That minute advantageous 

 differences will secure survival no one can doubt, but 

 it must be remembered that the variations which con- 

 stitute evolution have been in a vast number of cases 

 too minute to be useful. But the general question is 

 not affected by the supposition that advantageous va- 

 riations may be sometimes minute. Minute or great, 

 they have to be assumed in the argument for selec- 

 tion ; and whether minute or great, they have a def- 

 inite cause. 



* * 



In conclusion of Part II. of this book, I trust that I 

 have adduced evidence to show that the stimuli of 

 chemical and physical forces, and also molar motion 

 or use or its absence, are abundantly sufficient to pro- 

 duce variations of all kinds in organic beings. The va- 

 riations may be in color, proportions, or details of struc- 

 ture, according to the conditions which are present. 



