No. 494] THE LAW OF GEMINATE SPECIES 



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This list may be greatly extended, but the series noted 

 will illustrate the point in question. Whenever a dis- 

 tinct and sharply defined barrier exists, geminate or 

 twin species may be found on the two sides of it, unless, 

 as sometimes happens, the species has failed to maintain 

 itself on one side of the barrier. So far as Panama is 

 concerned, we have evidence that the barrier was raised 

 near the end of Miocene time with no trace of subsequent 

 depression. We can thus form some estimate of the age 

 of separation in at least a small number of closely related 

 species. In this and similar cases it is not possible to con- 

 ceive of the formation of these species by sudden muta- 

 tion, or that they would retain their separate existence 

 were the element of segregation removed. While segre- 

 gation or isolation is not a force, and perhaps not strictly 

 a cause in species formation, it is a factor which appar- 

 ently can never be absent, if the species retains its inde- 

 pendent existence. 



There is no doubt that the distribution of higher ani- 

 mals in general is in accord with "Jordan's Law." Ex- 

 amples by the thousand come up from every hand. If we 

 had a hundredth part of the amount of available evidence 

 in support of mutation theories, these theories would pass 

 from the realm of hypothesis into that of fact. But the 

 application of this law or rule to plants and to one-celled 

 animals has been questioned. So far as rhizopods are 

 concerned, Dr. Kofoid finds that the species are in general 

 sharply defined and of the widest distribution in the sea, 

 so that we can hardly state laws as denning their geo- 

 graphical distribution. To these minute floating animals, 

 the sea scarcely offers barriers at all, and the recognized 

 species do not seem to be products of geographical iso- 



