OF THE AMAZON VALLEY. 77 
tions being of uncertain character, it seems evident that when the 
mimicry is complete the indeterminate and intermediate grades of 
variation have been eliminated by insectivorous animals. The 
Leptalides have probably been subjected to this untiring persecu- 
tion, even to the verge of extermination, in each successive gene- 
ration throughout an immense lapse of time. By living in the 
vicinity of other species free from the same persecution, and being 
already similar to them in general appearance, this latter being 
inherited through a long line of ancestors which have been more 
or less subjected to similar conditions, such of their varieties as 
nearest resembled the protected species would escape destruction 
and thus alone propagate their kind. General resemblances 
between forms may be owing in some cases to similar habits, or 
to such general causes as have produced what are called recurrent 
animal forms ; this general resemblance must exist before the 
causes which produce mimetic analogies can come into play. It 
must be remarked that some of the exact resemblances already 
alluded to between species of Heliconine seem not to be owing to 
the adaptation of one to the other, but rather (as they have a real 
affinity, the genera to which they belong being throughout very 
similar in colours and all equally flourishing) to the similar adap- 
tation of all to the same external local conditions. The check 
which acts by destroying the indeterminate variations in these 
cases would not be the same as in Leptalis; in other respects, 
however, the same law of nature appears, namely, the selection of 
one or more distinct varieties by the elimination of intermediate 
gradations. It may be remarked also that a mimetic species need 
not always be a rare one, although that is very generally the case: 
it may be prolific, or its persecution may be intermitted when 
the disguise is complete. The operation of checks successively 
eliminating variations unfavourable to a species, as thus explained, 
produces the impression of a stimulus impelling an advance of 
organization in a special direction. This apparent direct advance 
suggests the only other scientific explanation that could be sug- 
gested, viz. the operation of volition or an innate tendency in the 
creatures themselves to become gradually assimilated to other 
forms, and thus acquire a disguise favourable to the species. On 
examination, however, this explanation is found to be untenable, 
and the appearances which suggest it illusory. Those who earnestly 
desire a rational explanation must arrive at the conclusion that 
these apparently miraculous but always beautiful and wonderful 
mimetic resemblances, like every other kind of adaptation in beings, 
are brought about by causes similar to those here discussed. 
