DISCUSSION OF DIAGRAMS 413 



species and varieties have been studied extensively by Merriam, Miller, 

 Osgood, and other manunalogists, but the characters which have been 

 used to distinguish them have not yet been analyzed in comparison with 

 the characters which are employed by paleontologists to distinguish the 

 ascending geologic mutations and species. 



Discussion of the illustrative Diagrams 



The accompanying diagrams present in graphic form this future prob- 

 lem for the cooperation of the zoologist and paleontologist who have made 

 observations independently hitherto without comparison of their separate 

 results. The very' circumstance that these facts have been garnered inde- 

 pendently by workers in two fields of observation, with no purpose but 

 the close definition and distinction of nearly related forms, renders all 

 the more valuable any results which may be obtained in the future. In 

 other words, the facts have been garnered both in zoology and vertebrate 

 paleontology without that bias or preconceived theory which often influ- 

 ences us to observe some facts rather than others. 



The problem of this comparison between zoologic and paleontologic 

 series is complicated by th^e fact that all zoologic series which are of wide 

 geographic range must necessarily be widely separated in time as well as 

 in space. For example, the genus Peromyscus as studied by Osgood 

 (1909) presents a continuous series of transitions in color and form 

 from the types which we observe on the Isthmus of Tehuantepec to those 

 we observe in Alaska. In the northerly region we find a larger animal, 

 with a relatively longer tail and a skull which may be somewhat longer 

 or more dolichocephalic. The genus Perognathus offers better examples 

 of skull change in connection with geographic distribution, for instance, 

 in tlie form of the interparietal bone. The wide distribution of Pero- 

 myscus may have taken place from some common center during post- 

 Glacial time, that is, during the last 20,000 years. In this period there 

 has been both a space evolution and a time evolution, the latter being 

 comparable to that which would be observed in a geologic series. This 

 principle is clearly brought out in the accompanying diagram, in which 

 A represents the stem or central form from which the geographic races 

 have been given off., which in course of time, undergoing an evolution in 

 time of from 5,000 to 40,000 years, diverge from the parent form (A) 

 precisely like the geologic or time series. Forty thousand years may have 

 elapsed since a geographic species or subspecies separated off from the 

 parent form. Even in geologic estimates 40,000 years is an appreciable 

 interval, in which new allometrons may arise and new rectigradations 

 may appear. 



XXIX— Bull. Geol. Soc. Am., Vol. 25, 1913 



