xxi. 7 DEFINITION OF A EUTHERIAN 575 



we can attempt some grouping of the great list of orders, before deal- 

 ing with them individually. 



6. Definition of a eutherian (placental) mammal 



The distinguishing characteristics of placentals are often listed 

 somewhat as follows : The young are retained for a considerable time 

 in the uterus and nourished by means of an allantoic placenta; there 

 is no pouch or epipubic bones. In the skull there is usually a separate 

 optic foramen, no palatal vacuities, and no in-turned angle of the jaw. 

 The tympanic bone is either ring-like or forms a bulla, there is never 

 an alisphenoid bulla. The brain has large cerebral hemispheres con- 

 nected by a corpus callosum. There is no cloaca. The dental formula 



3.1.4.3 

 is , or some number reduced from this. 



3-M-3 



Many of these are obviously small points of formal definition, arti- 

 ficially abstracted for the purpose of classification. They are not really 

 satisfactory as a definition of the life of a placental, such as we may 

 hope to have in a more developed biology. The early population of 

 Cretaceous insectivores presumably possessed most of these features, 

 and showed characteristics that are common to all the earlier mam- 

 mals. Among these we may list (1) small size; (2) short legs with a 

 plantigrade type of foot, having five digits; (3) the full eutherian 



3.1.4.3 



number of teeth , the molariform teeth being based on the 



_ 3-I-4-3 8 



tuberculo-sectorial pattern (p. 549); (4) long face and tubular skull, 

 enclosing a relatively small brain. 



7. Evolutionary trends of eutherians 



In the descendants of these early mammals we can recognize changes 

 in each of these four sets of characters ; changes occurring, indepen- 

 dently, in some members at least of all the later lines. (1) Many of 

 the mammals became larger. Increase in size seems to be advantageous 

 to many animal types and may be connected with the advantages of a 

 large brain storing much information during the life of the animal 

 and so allowing slow reproduction. It is presumably especially so for 

 warm-blooded animals in cold climates, since it reduces the relative 

 area of heat loss, though also introducing new problems of obtaining 

 adequate amounts of food. This may have to be finely ground by tooth- 

 surfaces whose increase with size is less rapid than that of the weight of 

 tissue they must support. (2) The limbs became longer and specialized 



