THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 27 



cause the complete or nearly complete exhaustion of a group. It would be 

 unfortunate, however, if an investigator were to be hasty in his conclusions 

 that the absence or diminished numbers of individuals or varieties of any 

 group in an observed portion of a unit implied such a condition. The 

 absence of fossils within the commonly restricted limits of any exposure of a 

 unit by no means implies the lack of an abundance of life during that interval 

 of time. The life of the ocean, fresh water, or land is far from uniformly 

 distributed, even in places where conditions are seemingly identical, and one 

 can not doubt that similar irregularities of distribution prevailed in past 

 time. Recognizing the eminently accidental way in which animal remains, 

 especially terrestrial forms, become preserved as fossils, a depression of 

 life should be considered as demonstrated only after the most thorough 

 search. Moreover, certain types of life may be driven out over large areas 

 and still exist in favorable localities elsewhere, as when the upper Silurian 

 fauna, depressed in the northern United States, found a "bay of refuge" in 

 the Gaspe region of Canada; nor can we doubt that,, though no great 

 number of crinoids have been found in Permian deposits, somewhere, as yet 

 unobserved, this branch of the animal kingdom maintained the stream of 

 life to reappear as an important factor in the Mesozoic. 



(g) THE INTERRELATIONS OF THE FAUNA. 



Xo small factor in the solution of a paleogeographic problem is the rela- 

 tion which each separate group or member of the fauna bears to others. 

 Parasitism renders some forms entirely dependent on the host, and un- 

 doubtedly the characters and habits of the host are affected by the parasites. 

 How important this is has been amply demonstrated in modern times, where 

 diseases carried by parasites have depopulated whole areas. The sleeping- 

 sickness trypanosome has practically extinguished human life in parts of 

 Africa by causing death or migration; the rinderpest wrought havoc with 

 the game in Africa; the Texas fever killed whole herds of animals in our 

 own Southwest; parallels to such extreme cases undoubtedly occurred in 

 past time and may have been the cause of the extinction or change of many 

 forms of life, both vertebrate and invertebrate. But it is not only in the 

 extreme cases that parasitism has had its effect. Groups large and small 

 have changed their habits and form by the assumption of parasitic habits 

 which were fatal to neither parasite nor host. 



Commensalism plays an important part in the modification of structure 

 and habits and was equally influential in the past. Some forms can only 

 live in connection with other forms, as in the body-cavities of other animals 

 or with them in caves, burrows, holes, etc. The little fish Fierasfer lives 

 in the branchial chamber of the sea-cucumber, and certain sponges grow 

 only on the backs and legs of certain crabs; the leptoline Hydractina grows 

 on the shells of hermit crabs, etc. In these cases one form does not prey 



