i 4 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



pialia indicates that the primitive types were all in- 

 sectivorous, and possessed a larger number of molars 

 than any of the recent forms. The latter have then 

 followed the same course as the placentals in the re- 

 duction of the number of teeth and specialization of 

 those that remain. 



Coming to the Monodelphia, the increase in the 

 size and complication of the brain, both of the cere- 

 bellum 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 pas- 

 sage of time. The successive changes in the structure 

 of the feet are all in one direction, viz., in the reduc- 

 tion of the number of the toes, the elevation of the 

 heel, and the creation of tongue and groove joints 

 where plain surfaces had previously existed. The 

 diminution in the number of toes might be regarded 

 as a degeneracy, but the loss is accompanied by a pro- 

 portional 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 

 extension of the second row of bones on the first, to the 

 inner side. In the highest and latest orders this pro- 

 cess 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 ("troch- 

 lear crest") in the elbow-joint. In the vertebrae the 

 development of the interlocking zygapophysial articu- 

 lations is a clear advance. 



Progress is generally noticeable in the dental struc- 



1 See Natiiralist, Jan., 1885, p. 55. 



