I5O THE METHOD OF DARWIN. 



of the effects. Sooner or later a cause is ob- 

 served to produce the effect of which an ex- 

 planation is desired; then by an induction the 

 cause is applied to the whole class of effects, 

 and the induction is established by subsequent 

 deduction and verification. 



One of the most interesting examples of 

 reasoning recorded in Darwin's works is that 

 concerning the colors of caterpillars. Although 

 Darwin put the problem and Wallace solved it, 

 it is inserted here because it illustrates the 

 devices used to secure explanations of facts. 

 Darwin had undertaken to explain the beauty 

 of butterflies by the principle of sexual selec- 

 tion, but knew that it was foolish to think that 

 the beauty of the mature animal was thus ex- 

 plained unless the equally beautiful colors of 

 many caterpillars, in whose case sexual selec- 

 tion certainly could not act, were explained in 

 some special way. 1 Here was another set of 

 facts that could not be reduced under his 

 theory; and again appears the almost insuper- 

 able difficulty of passing directly from facts to 

 their causes. He whom many regarded as a 

 master in the art of "wriggling" was unable to 

 devise an explanation, and appealed to Wallace, 



1 Descent of Man, etc., Vol. I. pp. 202-204 ; Life and Letters, 

 Vol. II. p. 276. 



