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OF THE AMAZON VALLEY. 
511 
prived of them. It is not unreasonable to suppose that some species are taken by in- 
sectivorous animals, whilst others flying in company with them are avoided. I could 
not, from their excessive scarcity, ascertain on the spot that the Leptalides were thus 
picked out. I noticed, however, that other genera of their family (Pieridce) were much 
persecuted. We have proof, in the case of Sand- Wasps, which provision their nests 
with insects, that a single species is very generally selected out of numbers, even of the 
same genus, existing in the same locality. I was quite convinced in the case of Career is 
binoclis of South America, which destroys numbers oi&Megalostomis (family ClythncUs), 
that the great rarity of the Beetle was owing to its serving as prey to the Cerceris. "We 
cannot point out all the conditions of life of each species concerned in these mimetic 
analogies. All that we can say is, that some species show, by their great abundance in 
the adult state, that during this period, before they propagate their kind, they enjoy by 
some means immunity from effective persecution, and that it is therefore an advantage 
to others not so fortunate, and otherwise unprovided for, if they are so like as to be 
mistaken for them. 
The process by which a mimetic analogy is brought about in nature is a problem 
which involves that of the origin of all species and all adaptations. "What I have pre- 
viously said regarding the variation of species, and the segregation of local races from 
variations, the change of species of Heliconidce from one locality to another, and the 
probable vital necessity of their counterfeits which accompany them keeping to the 
exact imitation in each locality, has prepared the way to the explanation I have to give. 
In the cases of local variation of the Heliconidce, there was nothing, as before remarked, 
very apparent in the conditions of the localities to show why one or more of the varieties 
should prevail in each over their kindred varieties. There was nothing to show plainly 
that any cause of the formation of local varieties existed, other than the direct action of 
physical conditions on the individuals, although this might be seen to be clearly incom- 
petent to explain the occurrence of several varieties of the same species in one locality. 
"We could only conclude, from the way in which the varieties occur in nature, as described 
in the case of Ilechanitis Polymnia, that the local conditions favoured the increase of one 
or more varieties in a district at the expense of the others — the selected ones being 
different in different districts. What these conditions were, or have been, was not revealed 
by the facts. With the mimetic species Leptalis Theono'e the case is different. We see 
here a similar segregation of local forms to that of Mechanitis Folymnia ; but we believe 
we know the conditions of life of the species, and find that they vary from one locality to 
another. The existence of the species, in each locality, is seen to depend on its form and 
colours, or dress, being assimilated to those of the Ithomia of the same district, which 
Ithomice are changed from place to place, such assimilation being apparently its only 
means of escaping extermination by insectivorous animals. Thus we have here the 
reason why local races are formed out of the natural variations of a species : the question 
then remains, how is this brought about ? 
The explanation of this seems to be quite clear on the theory of natural selection, as 
recently expounded by Mr. Darwin in the * Origin of Species.' The local varieties or 
cannot be supposed to have been formed by the direct action of physical condition s 
races 
VOL. XXIII. 
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