4C6 ON SUPPOSED BOTANICAL PROOFS OF [VII. 



words, the Ivy-shoot reacts under strong light with the pro- 

 duction of leaves and under weak light with the production of 

 roots, just as litmus-paper becomes red with an acid and blue 

 with an alkali. The physical nature of the Ivy-shoot was 

 present before the production of either structure, and was no 

 more due to the action of light itself, than the physical nature 

 of litmus-paper is due to an acid or an alkali. But this is quite 

 consistent with the possession of a physical nature which reacts 

 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 trans- 

 ferred 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 surroundings. 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 re- 

 actions 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^ upon the colours of certain caterpillars have distinctly 

 shown that some species possess the possibility of develop- 



^ [Dr. Weismann is here alluding to experiments upon the larvae of 

 Rinnia Crataegata. 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 



