OF THE AMAZON VALLEY. 



75 



which mimic strangely various bees, wasps, and other Hymeno- 

 pterous and Dipterous insects. The Heliconince, however, mimic 

 each other to a great extent — the parallel species belonging to 

 quite distinct "gjjjft eya. ' 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 of 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_£arieJ zg: 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 Heliconince, 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 



