394 ON THE SUPPOSED BOTANICAL PROOFS OF THE 



differently under the two different conditions afforded by light and 

 shade. 



No one would think of bringing forward the changes in the 

 colour of the green frog (Hyla) as a proof of the power of direct 

 influences in causing structural modifications in the animal body. 

 The frog is light green when it is resting upon green leaves, but it 

 becomes dark brown or nearly black when transferred to dark 

 surroundings. This is an obvious instance of adaptation, for the 

 changes in the colour of the frog depend upon a complex reflex 

 mechanism. The changes in the shape of the chromatophores of 

 the skin are not produced by the direct influence of the different 

 rays of light upon the body-surface, but in consequence of the 

 action of these rays upon the retina. Blind frogs do not react 

 under the changes of light. Hence it is impossible that any one 

 can maintain that the skin of the frog has gained its green colour 

 as the direct result of the green light reflected from its usual sur- 

 roundings. It must be admitted that in this and in all similar 

 cases, there is only one possible explanation, viz. an appeal to the 

 operation of natural selection. It may be objected that we are not 

 here dealing, as in the Thuja and Ivy, with changes in the course 

 of ontogenetic development following upon the occurrence of this or 

 that external condition, but only with the different reactions of 

 a mature organism. But nevertheless, cases of the former kind 

 appear to be also present in the animal kingdom. 



Thus the very careful and extensive investigations of Poulton 1 upon 

 the colours of certain caterpillars have distinctly shown that some 

 species possess the possibility of development 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 abundance 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 



[ Dr. Weismann is here alluding to experiments upon the larvae of Stnnia Cra- 

 taeyata. A short account of the results will be found in the Report of the British 

 Association at Manchester (1887), and in 'Nature,' vol. 36, p. 594. I have now 

 obtained similar results with many other species (see Trans. Ent. Soc., Loud. 1888, 

 p. 553) ; but many of the results are as yet unpublished. E. B. P.] 



