Parasites and their Belations to other Animals. 89 



fauna, as bearing, first, on the distinct creation of types of 

 animals, and, second, on the local creation of these types 

 wherever they are found. In the general fauna of the earth, 

 the fact that totally distinct genera and species exist, exposed 

 to the same external influences, is a strong argument against 

 their being the results of modifications of a single type, and 

 in favour of their having been created as we now see theni. 

 The same is true as regards these parasites ; we have dif- 

 ferent 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 would expect to find a uniformity in the special character 

 of parasites in all the species of the genera of that family. 

 For instance, if the SciuridaB 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 expe- 

 rience goes, is not true ; for though in many cases cer- 

 tain 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 own peculiar species of para- 

 sites. 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 can- 

 not understand that the same species of parasite should be 

 found in widely diff^erent families, of entirely dissimilar 

 habits ; yet such is the fact, and it is not reconcileable with 

 the hypothesis of a successive production of types by a series 

 of changes of their structure ; on the contrary, it would go to 

 shew that the existing specific types were as such created. 



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

 that the existence of the world's animals has not that com- 

 monness which might be supposed ; they have relations of 

 a local nature, connected with a remarkable diversity of 

 forms. At any rate, this fact is certain, that each particular 

 region has a marked tendency to have its own peculiar 

 fauna. Climate has undoubtedly a great influence on the 

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

 by impassable barriers, a tendency to similar animal produc- 



