Parasites and Geographical Distribution

Parasites and Geographical Distribution. The value of parasites of different animals for the investigation of the geographical distribution of the latter has been pointed out recently by H. von Ihering. I The object of modern zoogeographical research is chiefly to trace the origin of the different forms of life, and, with respect to the fauna of a limited section of the earth's surface, it is important to settle the question whether the inhabitants originated there, or wNhether they immigrated from other parts, and, in the latter case, whence they came. Von Ihering studies the present South American fauna from this point of view, and points out that, amionlg the faulila of this continent, we can distingoiish two chief elements: the one is peculiar to it, that is to say, was present there before the second half of the Tertiary, while the other immigrated from the North, after the Miocene. Then he proceeds to demonstrate that the parasitic worms found in these two groups of animals exhibit peculiar differences, so that it is possible, under certain circumstances, to cdraw the opposite conclusion that the parasites of a certain species of animal indicate, whether the latter belongs originally to South America, or whether it imimigrated in the later Tertiary. The instances quoted are taken chiefly from among Mammals and Birds, but it is evident that also other grouLps may furnish examples. In conclusion, von Iherinog condenses his results in three funclamental "biological laws," which we reproduce here, freely translated (i) Land animals, even if they migrate over a large extent of territory do not lose the parasitic wvorms peculiar to them because the lower animals wNlhich serve as intermediate hosts offer everywhere analogous conditions, provided everything else remains unchanged. Althotugh, in new areas of distribution, some new parasites may be aclecl, the old conditions largely remain unchanged, wh ich1 is very evident in South America, where the parasitic worms of the holarctic region are not found with the indigenous (autochthon) mammals or birds, but only with the strangers (heterochthon) that immigrated at a late period.