362 TROPICAL NATURE v 
protective, serving as a warning of their uneatableness.! On 
our theory none of these colours offer any difficulty. Con- 
spicuousness being useful, every variation tending to brighter 
and purer colours was selected ; the result being the beautiful 
variety and contrast we find. 
Imitative Warning Colours—the Theory of Mimicry 
We now come to those groups which gain protection solely 
by being mistaken for some of these brilliantly coloured but 
uneatable creatures, and here a difficulty really exists, and to 
many minds is so great as to be insuperable. It will be well 
therefore to endeavour to explain how the resemblance in 
question may have been brought about. 
The most difficult case, and the one which may be taken 
asa type of the whole class, is that of the genus Leptalis (a 
group of South American butterflies allied to our common 
white and yellow kinds), many of the larger species of which 
are still white or yellow, and which are all eatable by birds 
and other insectivorous creatures. But there are also a 
number of species of Leptalis, which are brilliantly red, 
yellow, and black, and which, band for band and spot for 
spot, resemble some one of the Danaidew or Heliconide which 
inhabit the same district and which are nauseous and uneat- 
able. Now the usual difficulty is, that a slight approach to 
one of these protected butterflies would be of no use, while a 
greater sudden variation is not admissible on the theory of 
gradual change by indefinite slight variations. This objection 
depends almost wholly on the supposition that, when the first 
steps towards mimicry occurred, the South American Danaidze 
were what they are now; while the ancestors of the Leptalides 
were like the ordinary white or yellow Pierids to which they 
are allied. But the Danaioid butterflies of South America are 
so immensely numerous and so greatly varied, not only in colour 
but in structure, that we may be sure they are of vast antiquity 
and have undergone great modification. A large number of 
them, however, are still of comparatively plain colours, often 
rendered extremely elegant by the delicate transparency of 
the wing membrane, but otherwise not at all conspicuous. 
1 This has since been found to be the case by Professor Herdman (Trans. 
Biol. Soc. Liverpool, vol. iv. p. 150). 
