62 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



great a difference between two specimens referred to the same species as 

 between two nominal species. Unless one is prepared to form a different 

 species for almost every individual, it is impossible to do more than to 

 make a tentative and more or less arbitrary arrangement. 



It cannot be supposed that eighteen or twenty species of a single mam- 

 malian genus should have co-existed at the same time and place; the diffi- 

 culty lies in determining how many of these forms were actually contem- 

 poraneous, or, in other words, to distinguish between variants and 

 mutants. Probably, the number of species to be recognized will be 

 materially reduced as the result of future investigations, for several of those 

 already proposed have been founded upon insufficient material and almost 

 all upon variations of the teeth only. 



It is significant of the rapid modification which Proterotherium was 

 undergoing in Santa Cruz times, that the variations appear to be largely 

 hap-hazard and that the species, or varieties, do not obviously fall into 

 groups, or subgenera, each group distinguished by some relatively impor- 

 tant characteristic. With great constancy in general plan, almost every 

 element of the tooth-pattern is subject to many minor variations, and 

 these variations do not occur together in a definite manner, but in all sorts 

 of combinations. It is as though some substantial improvement were 

 being sought by the method of "trial knd error." 



In a recent paper on the black bear of Labrador, Dr. J. A. Allen has 

 described a somewhat similar case among existing mammals. " Individ- 

 ual variation is especially manifest in the size and form of M-, which varies 

 greatly in skulls otherwise similar in general size and form. This tooth 

 varies in length, in specimens that seem unquestionably of the same sex, 

 from 22 to 27 mm., or about 20 per cent, of the mean. While these 

 variations are evident from the table of measurements, they are far more 

 impressive when the actual teeth are compared, since the shape of the 

 tooth varies as much as the size, especially in the development of the 

 'keel' portion. This is usually about one-third the length of the tooth, 

 but may be only one-fourth as long. The relation of width to length is 

 also markedly variable" (Allen, '10, pp. 3-5). 



