64 Studies in the Theory of Descent. 



of life as that in which they appeared in their pro- 

 genitors. The truth of this principle has been 

 firmly established, instances being known in which 

 both the first appearance of a new (especially 

 pathological) character and its transmission through 

 several generations has been observed. Seasonally 

 dimorphic butterflies also furnish a further valuable 

 proof of this principle, since they show that not 

 only variations which arise suddenly (and which 

 are therefore probably due to purely innate causes) 

 follow this mode of inheritance, but also that cha- 

 racters gradually called forth by the influence of 

 external conditions and accumulating from genera- 

 tion to generation, are only inherited at that period 

 of life in which these conditions were or are effec- 

 tive. In all seasonally dimorphic butterflies which 

 I have been able to examine closely, I found the 

 caterpillars of the summer and winter broods to be 

 perfectly identical. The influences which, by 

 acting on the pupae, split up the imagines into 

 two climatic forms, were thus without effect on 

 the earlier stages of development. I may specially 

 mention that the caterpillars, as well as the pupae 

 and eggs of A. Levana, are perfectly alike both in 

 the summer and winter forms ; and the same is the 

 case in the corresponding stages of P. Napi and 

 P. Bryonice. 



I shall not here attempt to enter more deeply 

 into the nature of the phenomena of inheritance. 

 It is sufficient to have confirmed the law that 



