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THE AMERICAN NATURALIST [Vol. XLI 



physical barriers, which determine their range, and which have 

 been factors in their formation. It may be claimed that such 

 conditions are virtually universal. In a few cases, a species ranges 

 widely over the earth, showing little change in varying conditions 

 and little susceptibility to the results of isolation. In other cases, 

 there is some possibility that sahations, or suddenly appearing 

 characters, may give rise to a new species within the territory 

 already occupied by the parent form. But these cases are so 

 rare that in ornithology, mammalogy, herpetology, conchology 

 and entomology, they are treated as negligible quantities. In 

 the distribution of fishes the same rules hold good, but as the 

 material for study is relatively far less extensive and less perfectly 

 preserved than with birds and insects, we have correspondingly 

 less certainty as to the actual traits of species and subspecies, 

 and the actual relation of these to the intervening barriers." 



President Jordan summarizes the distribution of species in a 

 law, as follows: "Given any species in any region, the nearest 

 related species is not likely to be found in the same region nor 

 in a remote region, but in a neighboring district separated from 

 the first l)y a barrier of some sort." That the intent of the law 

 involves both animal and vegetable kingdoms seems clear from 



President Jordan says that his conclusions, much as they differ 

 from a priori judgments or the results of experiment, are the 

 unavoidable outcome of the study of distribution, and that they 

 are as a matter of fact "accepted as self-evident by every com- 

 petent student of species or of the geographical distribution of 



Taking the facts of animal geography as they appear in these 

 several essays, typical of a larger number which might be cited, 

 we may say that as a whole they militate against the operation 

 of Mutation in any wide sense in the animal kingdom. This 

 conclusion is not prompted by the attitude of certain of the zoolo- 

 gists mentioned, who seem to have ma<le l)iif a curxny study of 



