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In the general Fauna of the earth, the fact that totally distinct 

 genera and species exist, exposed to the same external influ- 

 ences, is a strong argument against their being the results of 

 modifications of a single type, and in favor of their having been 

 created as v/e now see them. The same is true as regards these 

 Parasites ; we have different species, and even different genera, 

 upon a single animal, all exposed to the same external changes. 

 If genera and species are mere modifications of a primitive 

 family type, we should expect to find a uniformity in the special 

 characters of the parasites on all the species of the genera of that 

 family. For instance, if the Sciurids are but modifications of a 

 primitive type-Squirrel, we should expect to find certain parasites 

 common to all, with a uniform specific character, without widely 

 separated genera; which, as far as his experience goes, is not 

 true ; for though in many cases certain species of parasites are 

 common to the whole family, yet there is an evident tendency 

 for each species of the higher animals to have its peculiar species 

 of parasite. Though we can easily imagine that the same 

 species may be found in mammals and birds of the same family, 

 with similar habits, and associating together, we cannot under- 

 stand that the same species of parasite should be found in widely 

 different families, of entirely dissimilar habits ; yet such is the 

 fact, and it is not reconcilable with the hypothesis of a successive 

 production of types by a series of metamorphotic changes ; on 

 the contrary, it would go to show that the existing specific types 

 were as such created. 



As to the local creation of genera and species, we know that 

 the world's animals have not that commonness which might be 

 supposed ; they have relations of a local nature, connected with 

 a remarkable diversity of forms. At any rate, this fact is cer- 

 tain,* that each region shows a marked tendency to have its 

 peculiar Fauna. Climate has undoubtedly a great influence on 

 the character of a Fauna ; we see in the same zone, separated 

 by impassable barriers, similar animal productions, though there 

 is at the same time sufficient diversity to exclude the idea of a 

 common origin; different countries have analogous, not identical, 

 species. Geological data, the history of the surface of the earth, 

 and our fast increasing knowledge of the intimate relations of 

 animals to the circumstances in which they live, all lead to the 



