ASPECTS OF MODERN BIOLOGY 543 



regions surrounding the Mediterranean there are dozens of species of 

 shells of the same general type, while C. Ivbrica itself is widely spread 

 over the whole of Europe. Moreover, this species lubrica, so constant 

 with us, is there much more variable, so that nine varieties have been 

 found in the British Islands alone. 



The presumption is, then, that the snail, like the flax, is of Old 

 World origin, and represents a comparatively recent invasion from the 

 ancient area of distribution. This is supported by the occurrence of 

 allied but distinct genera in Europe and adjacent regions. 



Further investigation reveals hundreds of other cases similar to 

 those of the snail and the flax, and so it becomes more and more prob- 

 able, finally practically certain, that we owe a considerable part of 

 our fauna and flora to the immigration of animals and plants which 

 has reached nearly their present condition on the other side of the 

 world. We ourselves, of course, belong in this category. 



Having arrived at this point of view, the subject must not be 

 dropped, but should be attacked from another side. If America has 

 been overrun by Old World types in comparatively recent times, it 

 should be possible to get some idea of the time of these invasions by 

 examining the fossils of various strata. Unfortunately, the paleon- 

 tological record is very imperfect, but it yet yields facts of prime impor- 

 tance. We find that certain types, living in Colorado to-day, have lived 

 here with only slight modifications for many thousands, perhaps some 

 millions, of years. Others are totally absent, so far as our information 

 goes, from the older Tertiary strata, but negative evidence of this 

 kind must always be received with reservations. Others, to-day only 

 found in Asia, Africa, Europe or South America, were conspicuous 

 members of the Colorado biota. Here we find facts which throw doubt 

 on some of our previous conclusions. The Equidae, or horse family, 

 have to-day numerous members in Africa and some in Asia, but none 

 whatever in America. Yet we have evidence from the fossils that there 

 were formerly horses in America, and that they actually evolved on 

 this continent. The disease-carrying tsetse flies are to-day exclusively 

 African and might well be thought a peculiar product of that conti- 

 nent, but a species has turned up in the Colorado Miocene! So with 

 other cases, all tending to show that it is not safe to assume without 

 question that the original center of a group is the region where it is 

 now most abundant and varied. We do not thereupon decide that the 

 evidence from present distribution is valueless ; in many more instances 

 it leads to exactly the same conclusions as might be derived from the 

 fossils; but we recognize the importance of supplementing one kind 

 of fact with another, and considering all together when forming con- 

 clusions. When using the paleontological evidence, we are struck by 

 the differences between the fossils of successive strata and are always 



