78 THE GENEALOGY OF THE VERTEBRATA. 



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. Poly- 

 mastodon (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 complication of the brain, both of the cerebellum 



and the hemispheres, is a remarkable evidence of 



advance. But one retrogressive line in this respect is 



known, viz., that of the order Amblypoda, 1 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 



have previously existed. 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 of the feet is in 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 mechanical arrangement, the 



change is clearly an advance. The same progressive 



improvement is seen in the development of distinct 



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



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



In the verterbrre the development of the interlocking 



zygapophysial articulations is a clear advance. 



Progress is generally noticeable in the dental struc- 



1 See American Naturalist, Jan., 1885, p. 55. 



62 



