32 THE AGE OF MAMMALS 



tation on another is plainl^y written. Similar, })ut less extreme examples 

 are known among the liigher placental mammals of the northern hemisphere, 

 in which the theoretical life phases are as follows: 



1. A primitive aml)ulatory phase of a small, slow-moving animal, of 

 insectivorous or omnivorous type, provided with claws. 



2. The transformation into an herbivorous, ambulatory type pro- 

 vided with more or less well-formed hoofs, adapted to terrestrial gait and 

 relatively swift movements. 



3. Partial adaptation of a slower gait, accompanied by the conversion 

 of the hoofs into clumsy claws, adapted to digging or tearing down the 

 smaller branches of trees, as in the larger sloths. This return of an ungu- 

 late or hoofed type back to an imitation clawed type, like that of the 

 large ground-living sloths, occurs several times independently among the 

 typically hoofed mammals, the most extreme case being that of the chali- 

 cotheres {Macr other ium), which were mistaken for giant sloths by Cuvier, 

 but which really show a regression to an older habit. (See Fig. 130.) 



The Law of Analogous Evolution 



All the modes of change described above are divergent, or tending to 

 separate animals from each other. If the surface of the earth were infi- 

 nitely varied, and if animals had an infinite variety of means of adaptation 

 to certain conditions, undoubtedly all families and genera of mammals 

 would be entirely dissimilar from each other, but in comparing the habi- 

 tats of mammals in different parts of the earth, among the diversities of 

 condition we find similarities or repetitions of similar environments: each 

 continent has its mountains, its hillsides, its plains, its pampas, river borders, 

 swamps, deserts, grazing grounds, forests, its open country. Again, 

 the modes of adaptation of the epidermis, of the teeth, feet, and limbs of 

 mammals are also limited. The ingenuity of Nature in adapting animals 

 to similar conditions is not infinite; the same devices are repeatedly em- 

 ployed by her to accomplish the same adaptive ends. 



This repetition or duplication of habitat in different parts of the earth 

 underlies the law of analogous evolution, because mammals in their adapta- 

 tions to similar conditions of habitat or environment in different parts 

 of the earth have repeatedly converged or come together in their external 

 and more or less in their internal form, as well as in separate structures. 

 As regards the similar molding of single organs in many independent 

 groups of mammals, one of the first to trace this law in detail was W. B. 

 Scott in his masterly paper of 1891.^ This process of the analogous fash- 

 ioning of animals which may be only remotely related or not at all related 

 to each other is known as homoplasy, parallelism, and convergence. Thus 



' Scott, W. B., On tho Modo of Evolution in the Mammalia and on some of the Factors 

 in the Evolution of the Mammalia. Jour. Morphol., Vol. V, 1891, no. 3, pp. 361-378, 378-402. 



