VII.] mANSMISSION OF ACQUIRED CHARACTERS. 407 



merit in two directions, and that the actual direction taken by 

 the individual is decided by the influence of external conditions. 

 Poulton surrounded certain larvae of Geometrae with an abund- 

 ance of dark branches, in addition to the leaves upon which 

 they fed. When such conditions prevailed from the beginning 

 of larval life, the caterpillars as they developed, gradually 

 assumed the dark colour of the twigs and branches upon which 

 they rested. When other larvae of the same species (and in 

 many experiments hatched from the same batch of eggs) were 

 similarly exposed to the green leaves of the same food-plant, 

 they did not indeed become bright green like the leaves, but 

 were invariably of a much lighter colour than the other larvae, 

 while many of them gained a brownish-green tint. The larvae 

 of Smerinthiis ocellatus^ also possess the power of assuming 

 different shades of green and of thus approaching, to some 

 extent, the green of the plant upon which they happen to live. 

 It is quite impossible to explain the phyletic development of 

 the green colour of these and other caterpillars as due to the 

 direct action upon the skin of the green light reflected from the 

 leaves upon which they sit. The impossibility of such an effect 

 was pointed out long ago by Darwin, and also followed from 

 my own investigations. Here, as in the other cases, the only 

 possible solution is afforded by natural selection. The colour 

 of the caterpillars has become gradually more and more per- 

 fectly adapted to the colour of the leaves,— and often to the 

 particular side of the leaves upon which these animals rest, 

 — not by the direct effect of reflected light, but by the selection 

 of those individuals which were best protected. Poulton's 

 experiments quoted above prove that certain species which 

 occur upon different plants with different colours (or even in 

 some cases upon the differently coloured parts of the same 

 plant), present us with a further compHcation in the process of 

 adaptation, inasmuch as each individual has acquired the power 



species (see ' The Colours of Animals, their Meaning and Use especially 

 considered in the case of Insects,' Internat. Sci. Ser. London, 1890, 

 pp. 146-157), E. B. P. 



[1 See the editorial notes by Raphael Meldola, in his translation of 

 Weismann's 'Studies in the Theory of Descent' ^he Essay on 'The 

 Origin of the Markings of Caterpillars,' pp. 241 and 306) : also E. B. 

 Poulton, in ' Proc. Roy. Soc.,' vol. xxxviii. pp. 296-314; and in ' Proc. 

 Roy. Soc.,' vol. xl. p. 135.— E. B. P.] 



