350 THE AMERICAN NATURALIST. [Vol. XXXVII. 



Parasites and Geographical Distribution. — The value of para- 

 sites of different animals for the investigation of the geographical 

 distribution of the latter has been pointed out recently by H. von 

 Ihering. 1 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 

 whether they immigrated from other parts, and, in the latter case, 



point of view, and points out that, among the fauna of this continent, 

 we can distinguish 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 draw the opposite conclusion 

 that the parasites of a certain species of animal indicate, whether the 

 latter belongs originally to South America, or whether it immigrated 

 in the later Tertiary. 



The instances quoted are taken chiefly from among Mammals and 

 Birds, but it is evident that also other groups may furnish examples. 



In conclusion, von Ihering condenses his results in three funda- 

 mental "biological laws," which we reproduce here, freely translated: 



(1) Land animals, even if they migrate over a large extent of 

 territory do not lose the parasitic worms peculiar to them because 

 the lower animals which serve as intermediate hosts offer everywhere 

 analogous conditions, provided everything else remains unchanged. 

 Although, in new areas of distribution, some new parasites may be 

 added, the old conditions largely remain unchanged, which 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. 



(2) Under these circumstances, helminthology becomes a valu- 

 able aid for the analytic method of zoogeography, and we may con- 

 fidently obtain by it important results as to the history of such groups 

 in which we do not possess satisfactory geological material, or in 

 which such material naturally cannot be expected. 



1 Ihering, H. von. Die Helminthen als Hilfsmittel der zoogeographischen For- 



