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PREFACE. XXXV 



Mr. Wallace ' has applied it to tlio extirpation of varieties ; and 

 as these do arise in a wild species, he shows how such deviation 

 from type may either tend to the destruction of a variety, or to 

 adapt a variety to some changes in surrounding conditions, under 

 which it is better calculated to exist, than the type-form from 

 which it deviated. 



No doubt the type-form of any species is that -which is best 

 adapted to the conditions under ^\'hich such species at the time 

 exists ; and as long as those conditions remain unchanged, so long 

 will the type remain ; all varieties departing therefrom being 

 in the same ratio less adapted to the en\droniug conditions of 

 existence. But if those conditions change, then the variety of the 

 species at an antecedent date and state of things may become 

 the type-form of the species at a later date, and in an altered 

 state of things. 



In his work ' On the Origin of Species by Natural Selection,'^ 

 Mr. Darwin more fully exemplifies, conjecturally, the reciprocal 

 influence of external conditions and inherent tendencies to variety, 

 in carrying on, as he believes, the deviations from type to specific 

 and hiorher decrees of difierence. 



All these, however, are conceptions of what may have, not 

 observations of Avhat have, originated a species. Applied to 

 the structures which differentiate Trocilodi/tes f'rf)m Homo} or 

 Chiromys from Lemur, "^ "^^^^J are powerless to explain them : and 

 the structural differences in these instances are greater than in 

 many other species maintaining their distinction by sexual in- 

 capacity to produce fertile hybrids. 



An innate tendency or susceptilnlity in an offspring to differ 

 from a parent is a fact of observation : when carried beyond a 

 certain point the issue is called, from its rarity, a ' monster.' But 

 this tendency and its results are independent of internal volitions 

 and external influences. 



' Proceedings of the Linnean Societ)', August 1858, p. 57. - Svo. 1859. 



= On the Classification and Geographical Distribution of the ^lammaha, Sro. 1859, 

 p. 92. * Transactions of the Zoological .Society, vol. v. p. 86. 



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