OF THE AMAZON VALLEY. 16 
which mimic strangely various bees, wasps, and other Hymeno- 
pterous and Dipterous insects. The Heliconine, however, mimic | 
each other to a great extent—the parallel species belonging to’ 
quite distinct genera. In this case also, when the species present 
local varieties, the analogues are modified in precisely the same 
way, so that two or three species belonging to different genera 
resemble each other so much that they ‘can scarcely be distin- 
guished except by their generic characters. Endless instances 
of these imitative resemblances occur in entomology. Why they 
are so plentiful and amazingly exact in insects, whilst so rare and 
vague in the higher animals, is perhaps owing to the higher 
degree of specialization attained by the insect class, which is 
shown also by the perfection of their adaptive structures and 
instincts. Their being more striking and numerous in tropical 
than in temperate countries is perhaps attributable to the more 
active competitive life and the more rapid succession of the gene- 
rations in the former than in the latter. The meaning of these 
analogies is not difficult to surmise. In the first place, they 
cannot be entirely the result of similarity of habits or external 
physical conditions necessitating similar external dress. They 
are of the same nature as the assimilation of an insect or other 
animal in superficial appearance to the vegetable or inorganic 
substance on which it lives. The likeness of a beetle or lizard 
to the bark cf the tree on which it crawls cannot be explained as 
an identical result produced by a common cause acting on the tree 
and the animal: one is evidently adapted to the other. The in- 
finite variety of resemblances between insects and plants or inor- 
ganic substances—between predaceous animals and their victims— 
the adaptation of organs or functions to the objects or habits they 
relate to—are all of the same nature. They are adaptations either 
of the whole outward dress or special parts, all having in view 
the welfare of the creatures that possess them. Every species in 
nature may be looked upon as maintaining its existence by virtue 
of some endowment enabling it to withstand the host of adverse 
circumstances by which it is surrounded. The means by which 
the existence of species is maintained are of endless diversity ; 
and amongst them may be reckoned the resemblance of an other- 
wise defenceless species to another whose flourishing race shows 
that it possesses peculiar advantages. The Heliconine, by the 
great number of their individuals, show themselves to be a 
favoured family. It is not easy to discover anything in their 
structure or habits which might give them an advantage. There 
