CHAP. IV Shifts for a Living 51 



" It appears to be certain that it is, the skin of the larva 

 which is influenced by surrounding colours during the 

 sensitive period, and it is probable that the effects are 

 wrought through the medium of the nervous system." 



Accepting the facts that caterpillars are subtly affected 

 by surrounding colours, so that the quiescent pupae har- 

 monise with their environment, and that the adjustment has 

 often protective value, we are led to inquire into the origin 

 of this sensitiveness. That the change of colour is 

 not a direct result of external influence is certain, but 

 of the physiological nature of the changes we know little 

 more than that it must be complex. It may be main- 

 tained, that " the existing colours and markings are at any 

 rate in part due to the accumulation through heredity 

 of the indirect influence of the environment, working 

 by means of the nervous system ; " " to which it may 

 be replied," Poulton continues, " that the whole use and 

 meaning' of the power of adjustment depends upon its 

 freedom during the life of the individual ; any hereditary 

 bias towards the colours of ancestors would at once destroy 

 the utility of the power, which is essentially an adaptation 

 to the fact that different individuals will probably meet with 

 different environments. As long ago as 1873 Proftssor 

 Meldola argued that this power of adjustment is adaptive, 

 and to be explained by the operation of natural selection." 

 Poulton's opinion seems to be, that the power of producing 

 variable colouring arose as a constitutional variation apart 

 from the influence of the environment, that the power was 

 fostered in the course of natural selection, and that its 

 limits were in the same way more or less defined in adapta- 

 tion to the most frequent habitat of the larvas before 

 and during pupation. The other theory is that the power 

 arose as the result of environmental influence, was accumu- 

 lated by inheritance throughout generations, and was fostered 

 like other profitable variations by natural selection. The 

 question is whether the power arose in direct relation 

 to environmental influence or not, whether external influence 

 was or was not a primary factor in evolving the power of 

 adapting colour, and in defining it within certain limits. 



