regions surrounding the Mediterranean there are dozens of species of shells of the same general type, while C. lubrica 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 probable, 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 paleontological record is very imperfect, but it yet yields facts of prime importance. 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 Equidæ, 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 continent, 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 conclusions. When using the paleontological evidence, we are struck by the differences between the fossils of successive strata and are always
