No. 594] 



NOTES AND LITERATURE 



370 



to breed, this is not so firmly established as to leave no doubt 

 about which is going and which is coming. 



Adams clearly stated that these criteria are for use where 

 "we do not have paleontological evidence in sufficient abundance 

 to materially aid us," but I confess To a feeling that we must 

 still depend on paleontology to give us the general laws of dis- 

 persal. It is for this reason that such papers as Matthew's seem 

 to me so helpful. I regret that the .uranisms in which I am most 

 interested did not leave more marks on the sands of time, but if 

 the great majority of mammalian u roups left records to show 

 that they followed a certain set of lines of dispersal and the end 

 result is of a given character, it seems worth while to compare 

 the end results of the dispersal of other forms with those of the 

 mammalian groups. If the comparison is close, the deduction 

 that the lines of dispersal also are comparable seems not unsafe. 

 It certainly seems unwise to construct trans-oceanic hrhk.-s 

 where conservative geologists say there could have been none, 

 especially if they must be made so tenuous that only insects, 

 spiders, snails, earthworms, fresh-water fishes and such small fry 

 can cross, mammals being forbidden. 



Several of the criteria given by Adams which appear to fail in 

 helping us discover the ancient centers of dispersal seem to be 

 indicators of present or potential centers. Thus the first one : a 

 region where there is a "great differentiation of type" within a 

 group would seem to be a region prepared to send members of 

 that group into all the world. If paths exist or chance inter- 

 venes, this group should be able to fit some, at least, of its many 

 different types into the new environments which it encounters in 

 its spread. If all the surrounding territory is already occupied 

 by more successful competitors, that merely means that there are 

 no paths for the dispersal of this group. One of the real, but 

 sometimes overlooked, barriers to faunal spread is the presence 

 of competitors. This, however, does not negate the idea that a 

 region of "great differentiation of type" is a potential center of 

 dispersal. 



In somewhat the same way the region of "dominance or great 

 abundance of individuals" is a potential center of dispersal. 

 The case is not so clear; but it would seem that where there is a 

 relatively large supply of individuals more could be spared for. 

 or would be forced into, colonization efforts than where there 

 are relatively few. 



This suggests that what was the center of greatest develop- 

 ment (especially in variety of types) became the center of dis- 



