1885.| Progressive and Retrogressive. 351 
The dentition of some of the Sirenia is like that of some of 
the Ungulata, especially of the suilline group, while in others the 
teeth consist of cylinders. In the Cetacea the molars of the old- 
est (Eocene and Miocene) types are but two-rooted and com- 
pressed, having much the form of the premolars of other Mam- 
malia. In existing forms a few have simple conical teeth, while 
in a considerable number teeth are entirely wanting. 
A review of the characters of the existing Mammalia as com- 
pared with those of their extinct ancestors displays a great deal 
of improvement in many ways, and but few instances of retro- 
gression. The succession in time of the Monotremata, the Mar- 
supialia, and the Monodelphia, is a succession of advance in all 
the characters of the soft parts and the skeleton which define 
them (see table of classification), As to the monotremes them- 
selves, it is more than probable that the order has degenerated in 
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 Eo- 
cene, is as specialized as the most specialized recent forms. The 
dentition of the Jurassic forms, Plagiaulax, etc., is quite special- 
ized also, but not more so than that of the kangaroos. The pre- 
molars 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 num- 
ber of the toes, the elevation of the heel and the creation of 
tongue and groove joints where plain surfaces has previously 
existed. The diminution in the number of toes might be re- 
garded 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 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 
1See NATURALIST, Jan., 1885, p. 55- 
