110 ACQUIRED CHARACTERS sec. 



Now, the larval stage of the cockchafer certainly repeats 

 a stage in the ancestral evolution. And this example shows 

 that the ancestral evolution must have been retarded under 

 unfavourable conditions of temperature or nutrition — in other 

 words, that the evolution of the type was, like the individual 

 growth itself, conditioned by warmth and nutrition. 



Moreover, it seems self-evident that a corresponding effect 

 must be ascribed to warmth alone, as is shown bv the influ- 

 ence of temperature on the period of development, in the most 

 various species, not only of larvae, but also of chrysalids, which 

 take no nourishment. I shall shortly have to speak at 

 greater length on the importance of warmth in the modification 

 of species. Previously I have to point out some facts which 

 show still further the influence of nutrition in the process. 



Every boy knows that the butterfly called the "brown bear," 

 Euprepia caja, can be reared in various varieties, according to 

 the different nourishment wdiich is supplied to the caterpillar. 

 Here again the varieties which arise through change of 

 food are not fortuitous but perfectly definite. For the differ- 

 ences of the markings which numerous such varieties in my 

 collection exhibit are not irregular, but reveal, as in other 

 cases, quite definite directions of modification. 



Other butterflies are also known on whose colourincr and 



o 



markings the food of the caterpillars has great influence. I 

 have special grounds for the conviction that many new species 

 have arisen through the caterpillars having been at one time 

 or another forced to accommodate themselves to a chancre of 

 food. In evidence of this is the fact that numerous very 

 slightly differing species of Vanessa, e.g. V. polychloros, xautho 

 nielas, 1. album, and urticic, lay their eggs on different plants. 

 It is natural to infer that in such cases the difference in 

 the quality of the food has been the cause of the origin of 

 different characters. 



I have to consider now a special example, which I have 



