Page:Biological Analogies in History.djvu/18

 sickness,' which in many large districts has killed out the entire population. Of course the development or the extension of the range of any such insects, and any one of many other causes which we see actually at work around us, would readily account for the destruction of some given species or even for the destruction of several species in a limited area of country.

When whole faunal groups die out, over large areas, the question is different, and may or may not be susceptible of explanation with the knowledge we actually possess. In the old arctogaeal continent, for instance, in what is now Europe, Asia, and North America, the glacial period made a complete, but of course explicable, change in the faunal life of the region. At one time the continent held a rich and varied fauna. Then a period of great cold supervened, and a different fauna succeeded the first. The explanation of the change is obvious.

But in many other cases we can not so much as hazard a guess at why a given change occurred. One of the most striking instances of these inexplicable changes is that afforded by the history of South America toward the close of the tertiary period. For ages South America had been an island by itself, cut off from North America at the very time that the latter was at least occasionally in land communication with Asia. During this time a very peculiar fauna grew up in South America, some of the types resembling nothing now existing, while others are recognizable as ancestral forms of the ant-eaters, sloths, and armadillos of to-day. It was a peculiar and diversified mammalian fauna, of, on the whole, rather small species, and without any representatives of the animals with which man has been most familiar during his career on this earth.