18 THE AGE OF MAMMALS 



Brachycephaly, a short, l)road-headed condition of the skull, charac- 

 teristic of some progressive forms. 



DoLiCHOCEPHALY, an elongate condition of the skull, especially of the 

 anterior or facial portion in front of the eyes, as in the horse (Equus) 

 and in the moose (Alcea). 



The rationale of these changes of proportion is very different in dif- 

 ferent skulls, so that we never can assume that long-headedness is due 

 to any single cause. In the horse long-headedness is a very ancient char- 

 acter; even the earliest known four-toed horses have quite elongate, or 

 at least mesaticephalic, skulls. The progressive elongation of the skull 

 in horses is apparently for two purposes: first, to facilitate reaching the 

 ground with the row of incisor or cropping teeth; second, and no less 

 important, to allow space in front of the eye sockets for the great rows of 

 elongate, or hypsodont, grinding teeth, the marvelous dental battery of 

 the horse. We might assume from these facts that long-headedness is 

 correlated with long teeth, but the giant pigs (elotheres) have still longer 

 and narrower skulls than the horse, yet all the teeth are brachyodont, or 

 short-crowned. Again, the elephant has extremely elongate or hypsodont 

 molar teeth, yet it possesses also the shortest, or most brachycephalic, 

 skull known among the Mammalia. 



Thus all kinds of combinations and changes of proportion occur in the 

 evolution of mammals. The correlation is not that of certain fixed types 

 of structure, but it is a correlation of perfect adaptations to different de- 

 mands brought about by the changes in habitat. 



II. Mammals and their Environment 



The fitness of mammals to their environment takes us back to another 

 line of thought in the history of palaeontology, in which we see that the 

 idea of the evolution of the environment, as revealed by the study of fossils 

 and the earth itself, grew step by step with the idea of the evolution of 

 mammalian life. It has become clear that there are two ways in which 

 mammals experience a change of environment: either through their own 

 migrations, or through "the migration of the environment itself," as Van 

 den Broeck has expressed it, in the successive historic changes of certain 

 parts of the earth in course of time. Thus if we imagine a family of mam- 

 mals residing continuously in the region now known as South Dakota, the 

 early humid environment has migrated to quite a different part of the 

 American continent, and an entirely new, arid environment has come in. 

 Sometimes mammals and their environment migrate together. This was 

 certainly the case during the Glacial Period, when physiographic condi- 

 tions, faunas, and floras all moved southward together before the advance 

 of the ice sheet, and again moved northward together as the ice retreated. 



