180 THE DOCTRINE OF DESCENT. 
ordinarily extensive; they are remarkable for their 
elongated wings, body, and antennz, and for the beauty 
of their colours. It might be imagined they were 
exposed to the persecutions of insectivorous birds and 
other animals ; but this is not the case, for they have a 
disagreeable smell, which, in all likelihood, renders them 
obnoxious. Their smell and flavour are thus a protection, 
as the birds and lizards who have once seized them by 
mistake are certain, ever after, to leave them unmolested. 
Now, as the insectivora do not test the individual case, 
but have adopted a general repugnance to the aspect of 
the Heliconida, if other butterflies resembled the Heli- 
conidz without possessing the bad smell, they would 
participate in the security to life enjoyed by the Heli- 
conidz in proportion as they approach their external 
appearance. This case has actually occurred, for Bates 
discovered a number of species of the otherwise very 
different genus, Leptalis, of which each almost undis- | 
tinguishably resembles one of the Heliconide both in 
colour and form. The Leptalidz have also adopted the 
flight of the Heliconidze, share their habitats, and, 
although without the offensive smell, fly about with 
impunity. This state of things would be impossible if 
the Leptalide were not considerably in the minority, so 
as to be in a measure hidden by the Heliconidz. 
Wallace has proved that species protected by mimicry 
of other animals are invariably in the minority, and 
often very rare in comparison with the species which 
they imitate. Neither the explanation that like condi- 
tions of life produced like results, nor the hypothesis 
that, in some cases at least, the mimicry consists in 
reversion to a common original species, is in any way 
