xlii MEMOIR. 
kinds of imitative resemblances belong to the same class of pheno- 
mena, and are subject tothe same explanation. All are adaptations, 
either of the whole outward dress or of special parts, 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 are of endless. 
diversity. Some are provided with special organs of offence, others. 
have passive means of holding their own in the battle of life. Great 
fecundity is generally of much avail, added to capabilities, active or 
passive, of wide dispersion ; so that when the species is extirpated 
in one part of its area of distribution, the place is refilled by 
migration of individuals from another part. A great number 
have means of concealment from their enemies, of one sort or 
other. Many are enabled to escape extermination, or obtain sub- 
sistence, by disguises of various kinds; amongst these must be 
reckoned the adaptive resemblance of an otherwise defenceless. 
species to one whose flourishing race shows that it enjoys peculiar 
advantages.” * 
Now what Bates did, as Darwin says in his review of the 
paper,t was to give to “these facts (of adaptive resemblance) the 
requisite touch of genius, and hit on the final cause of all this. 
mimicry.” 
“ The explanation seems to be quite clear on the theory of natural 
selection, as recently expounded by Mr. Darwin in the Ovigin of 
Species. The local varieties, or races, cannot be supposed to have 
been formed by the direct action of physical conditions on the 
individuals, because in limited districts, where these conditions are 
the same, the most widely contrasted varieties are found existing 
together, and it is inexplicable how they could have produced the 
nice adaptations which these diverse varieties exhibit. Neither can 
these adapted races, as before remarked, have originated in one 
generation by sorts, or a single act of variation in eachcase. It is. 
clear, therefore, that some other active principle must be here at 
work to draw out, as it were, steadily in certain directions, the 
suitable variations which arise, generation after generation, until 
forms have resulted which, like our races of Leptal:s Theonoé, are 
considerably different from their parent as well as their sister 
forms. 
* Trans. of Linn. Soc., p. 510. + Nat. Hist. Review, April 1863. 

q 
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