Further facts can be adduced in support of the view that in these processes it is the constitution of the species, and not the external agency (warmth), which plays the chief part. The latter, as Darwin has strikingly expressed it, rather performs the function of the spark which ignites a combustible substance, whilst the character of the combustion depends upon the nature of the explosive material. Were this not the case, increased warmth would always change a given colour[29] in the same manner in all butterflies, and would therefore always give rise to the production of the same colour. But this does not occur; _Polyommatus Phlæas_, for example, becoming black in the south, whilst the red-brown _Vanessa Urticæ_ becomes black in high northern latitudes, and many other cases well known to entomologists might be adduced.[30] It indeed appears that species of similar physical constitution, i.e., nearly allied species, under similar climatic influences, change in an analogous manner. A beautiful example of this is furnished by our _Pierinæ_. Most of the species display seasonal dimorphism; as, for instance, _Pieris Brassicæ_, _Rapæ_, _Napi_, _Krueperi_, and _Daplidice_, _Euchloe Belia_ and _Belemia_, and _Leucophasia Sinapis_, in all of which the difference between the winter and the summer forms is of a precisely similar nature. The former are characterized by a strong black dusting of the base of the wings, and by a blackish or green sprinkling of scales on the underside of the hind wings, while the latter have intensely black tips to the wings, and frequently also spots on the fore-wings. Nothing can prove more strikingly, however, that in such cases everything depends upon the physical constitution, than the fact that in the same species the males become changed in a different manner to the females. The parent-form of _Pieris Napi_ (var. _Bryoniæ_) offers an example. In all the _Pierinæ_ secondary sexual differences are found, the males being differently marked to the females; the species are thus sexually dimorphic. Now the male of the Alpine and Polar var. _Bryoniæ_, which I conceive to be the ancestral form, is scarcely to be distinguished, as has already been mentioned, from the male of our German winter form (_P. Napi_, var. _Vernalis_), whilst the female differs considerably.[31] The gradual climatic change which transformed the parent-form _Bryoniæ_ into _Napi_ has therefore exerted a much greater effect on the female than on the male. The external action on the two sexes was exactly the same, but the response of the organism was different, and the cause of the difference can only be sought for in the fine differences of physical constitution which distinguish the male from the female. If we are unable to define these differences precisely, we may nevertheless safely conclude from such observations that they exist.