348 THE STEUCTURAL EVIDENCE OF EVOLUTION. 



some respects in producing the existing types. The history of the 

 Marsupialia is not made out, but the earliest forms of which we 

 know the skeleton, Polymastodon (Cope) of the Lower Eocene, is as 

 specialized as the most specialized recent forms. The dentition of 

 the Jurassic forms, Plagiaulax, etc., is quite specialized also, but 

 not more so than that of the kangaroos. The premolars are more 

 specialized, the true molars less specialized than in those animals. 



Coming to the Monodelphia, the increase in the size and com- 

 plication of the brain, both of the cerebellum and the hemi- 

 spheres, is a remarkable evidence of advance. But one retro- 

 gressive line in this respect is known, viz., that of the order 

 Amblypoda,* where the brain has become relatively smaller with 

 the passage of time. The successive changes in the structure of 

 the feet are all in one direction, viz., in the reduction of the 

 number of the toes, the elevation of the heel, and the creation of 

 tongue and groove joints where plain surfaces had previously ex- 

 isted. The diminution in the number of toes might be regarded 

 as a degeneracy, but the loss is accompanied by a proportional 

 gain in the size of the toes that remain. In every respect the 

 progressive change in the feet is an advance. In the carpus and 

 tarsus we have a gradual rotation of the second row of bones on 

 the first, to the inner side. In the highest and latest orders this 

 process is most complete, and, as it results in a more perfect me- 

 chanical arrangement, the change is clearly an advance. Tlie 

 same progressive improvement is seen in the development of dis- 

 tinct facets in the cubito-carpal articulation, and of a tongue and 

 groove (" intertrochlear crest ") in the elbow-joint. In the ver- 

 tebrae the development of the interlocking zygapophysial articula- 

 tions is a clear advance. 



Progress is generally noticeable in the dental structures ; for, 

 unlike the marsupial line, the earliest dentitions are the most 

 simple, and the later the more complex. Some of the types re- 

 tain the primitive tritubercular molars, as the Centetidfe, shrews 

 and some lemurs, and many Carnivora, but the quadritubercular 

 and its derivative forms are by far the most common type in the 

 recent fauna. The forms that produced the complicated modifica- 

 tions in the Proboscidia and Diplarthra appeared latest in time, 

 and the most complex genera. Bos and Equus, the latest of all. 

 The extreme sectorial modifications of the tritubercular type, as 



* See "Naturalist," Jan., 1885, p. 55. 



