INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



The great family tree of the perissodactyls may be interpreted as shown below. 



Family tree of the perissodactyls 



39 



The mediportal structure, in which the skeleton 

 and limbs are adapted to moderate speed and weight, 

 embraces those intermediate stages in several different 

 families in which there was moderate body weight 

 and moderate speed, as in the tapirs. In the tapirs 

 this is the last term of evolution, but in the titanotheres 

 and in many rhinoceroses the mediportal stage is 

 simply a gateway to the graviportal stage, in which 

 the proportions of the limbs and trunk are adapted to 

 weight bearing, more or less rapid progression, and 

 active offense and defense. 



The interpretation of these phenomena of analogous, 

 parallel, and convergent evolution under the princi- 

 ple of adaptive radiation, presented on pages 121-127, 

 simplifies the problem of the anatomy of the group 

 as a whole as well as of the several adaptations 

 seen in the skull, skeleton, limbs, and teeth. Each 

 perissodactyl family appears to exhibit an innate 

 potentiality to evolve in many different directions 

 and thus to meet new conditions of life. In this 

 sense each family is plastic. Here we are not wit- 

 nessing the direct action of the environment: we 

 are witnessing the direct response of the organism, 

 through largely unknown causes, to develop its poten- 

 tial heredity characters along certain new lines. If 

 the supply of new potential characters is exhausted, 

 if a mechanical stage is reached out of which no addi- 

 tional stages can be developed, the animal will tend 

 to become extinct unless it can retire to the recesses 

 of the forests, as did the chalicotheres, and thus escape 

 a struggle for existence in competition with more 

 plastic forms, better adapted to the grazing life. The 

 interpretation of these processes, however, has been 



the most difficult and baffling of all the problems that 

 have arisen in the research made for this monograph. 

 The interpretation of the modes and causes of the 

 origin and evolution of new characters and of new 

 proportions in response to new conditions of life 

 (see pp. 834-849) is extremely difficult. Explanations 

 that at first seem obvious appear on close analysis 

 not to be explanations at all. As this monograph is 

 the most exhaustive and most detailed study thus far 

 made of any group of mammals it seems important 

 to show the bearing of all the observations on each of 



Figure 31. — Three types of teeth of members 

 of nine typical families of perissodactyls 



Bunoselenodont (A), bunolophodont (B), and lophodont (C) 

 types of teeth displayed in the short-crowned Cbrachyodont) 

 stage. 



the current theories of evolution. It appears that, 

 as is fully set forth in Chapter XI, we are still very 

 far from even a preliminary understanding of the 

 causes of many of the processes of mammalian 

 evolution. 



VELOCITY IN THE DEVELOPMENT OF CHARACTERS AND IN 

 PHYLOGENY 



The earliest explanations of evolution were purely 

 mechanical; we are now passing through a phase of 



