VERTEBRATES : BAOKBONE AND BRAIN 91 



the wall of the uterus would be of the greatest advan- 

 tage to the species. 



This is not the whole explanation ; other factors still 

 undiscovered were undoubtedly concerned. But even 

 this shows us that the internal development of the 

 young and the habit of suckling them was a logical re- 

 s\ilt of mammalian structure and position. The grand 

 results of this change we shall trace farther on. 



The changes from the lower true mammals to the 

 apes are of great interest, but we can notice only one 

 or two of the more important. The prosimii, or " half 

 apes," including the lemurs, are nearly, all arboreal 

 forms. Perhaps they were driven to this life by their 

 more powerful competitors. The arboreal life devel- 

 oped the fingers and toes, and most of these end, not 

 with a claw, but with a nail. The little group has 

 much diversity of structure, and at present finds its 

 home mainly in Madagascar ; though in earlier times 

 apparently occurring all over the globe. The brain is 

 more highly developed than in the average mammal, 

 but far inferior to that of the apes. They have a fairly 

 opposable thumb. 



The highest mammals are the primates. Their 

 characteristics are the following : Fingers and toes all 

 armed with nails, the eyes comparatively near together 

 and fully enclosed in a bony case. The cerebrum with 

 well-developed furrows covers the other portions of 

 the brain. There is but one pair of milk-glands, and 

 these on the breast. The differences between hand and 

 foot become most strongly marked by the " anthro- 

 poid " apes. These have become accustomed to an up- 

 right gait in their climbing ; hence the feet are used for 

 supporting the body and the hands for grasping. Both 



