242 SEXUAL DIMORPHISM 



edible species which resemble the other variety of the inedible 

 species. This supposition is so manifestly absurd and 

 impossible that no one who has realised it can continue to 

 regard natural selection alone as a sufficient explanation of 

 the refinements of mimicry to which I have alluded. 



That external conditions, whether of climate, food, or 

 light, have had an important influence in determining the 

 characters of the Heliconidse in South America, which are 

 there the forms imitated by other species, is indicated by the 

 statement of Bates that these species are peculiarly creatures 

 of the tropical forests, like the Platyrhine monkeys, the 

 arboreal Gallinacei (Penelopidse and Cracidse), the sloths, 

 and other animals of the same region. The Heliconidse are 

 most numerous where the forests are most extensive and the 

 climate most sultry and humid. Since the Heliconidse are 

 accompanied in their gregarious flocks by the imitating 

 forms, the latter have as a matter of fact been subjected to 

 the same influences. Where the sexes are dimorphic in the 

 imitating forms, the difference of conditions appears to 

 consist, as in the cases mentioned below, in the fact that the 

 males resort to sunny places in the daytime, and seek the 

 females only in the evening. 



The sexes of many butterflies and moths differ consider- 

 ably, although neither sex is mimetic. Darwin has discussed 

 these cases at some length, but it can scarcely be considered 

 that he has shown it to be very probable that the differences 

 are due to sexual selection. He admits that many serious 

 objections may be urged against this explanation. He points 

 out that in several cases the males and females of the same 

 species of butterfly are known to inhabit different stations, 

 the former commonly basking in the sunshine, the latter 

 haunting gloomy forests. 1 He considers that it is thus 



1 The passage to which Darwin refers in Bates' Naturalist on the 

 Amazo?is is worth quoting here : "It is a singular fact that with very few 



