On the Seasonal Dimorphism of Butterflies. 1 1 3 



not only is the different external appearance of 

 species as well as their physiological and biological 

 diversity thus explained, but it necessarily follows 

 therefrom, that different species must respond dif- 

 ferently to those external causes which tend to 

 produce a change in their form. 



Now, this last conclusion is equivalent to the 

 statement that every species, through its physical 

 constitution, (in the sense defined) is impressed 

 with certain fixed powers of variation, which are 

 evidently extraordinarily numerous in the case of 

 each species, but are not unlimited ; they permit 

 of a wide range for the action of natural selection, 

 but they also limit its functions, since they certainly 

 restrain the course of development, however wide 

 the latter may be. I have elsewhere previously 

 insisted 6 that too little is ascribed to the part played 

 by the physical constitution of species in the his- 

 tory of their transformation, when the course of 

 this transformation is attributed entirely to external 

 conditions. Darwin certainly admits the impor- 

 tance of this factor, but only so far as it concerns 

 the individual variation, the nature of which ap- 

 pears to him to depend on the physical constitution 

 of the species. I believe, however, that in this 

 directive influence lies the precise reason why, 

 under the most favourable external circumstances, 

 a bird can never become transformed into a mam- 



6 See my essay, " Uber die Berechtigung der Darwin'schen 

 Theorie." Leipzig, 1868. 



T 



