160 EVOLUTION AND GENETICS 



kangaroo and the opossum, and are viviparous. The young are, 

 however, usually without a placental connection with the mother, 

 and are in an imperfect state of development when born. They are 

 then carried in an abdominal pouch and are temporarily attached 

 to the long tubular teats by a special oral sucker. In the last 

 group occur those animals whose young are connected to the uter- 

 ine wall of the mother by a placenta. Through this the developing 

 animal receives nourishment from the blood of the parent until 

 birth. 



The Euthcria include most mammals. They are supposed to 

 have originated from the cynodont stem later than the Metatheria, 

 while the Prototheria are supposed to have sprung from an even 

 more primitive reptile. During the Tertiary they attained the 

 differentiation which they have since maintained. Some species 

 of immense size appeared, although in this direction the mammals 

 were surpassed by the reptiles ; the whales of the present are among 

 the largest animals ever developed. While this period in mam- 

 malian development produced some species belonging to existing 

 genera, all are now extinct. Most of the Tertiary mammals were 

 less closely related to existing species. 



Dispersal. In writing of the rise of the mammals during the 

 Tertiary, Lull calls attention to the similarity of the North Ameri- 

 can and European faunas, and to the existence of a land bridge 

 across what is now Bering Strait as indicative of their circumpolar 

 origin. He adds that the climate of this region was at first warm 

 and favorable and speaks as follows of the influences of the 

 period : 



"There is always a tendency on the part of every group of 

 animals, as their numbers increase, to spread from their ancient 

 home along lines of least resistance, provided no climatic or other 

 insuperable barriers are to be overcome, and that may well have 

 been one very potent cause for the southward migration of the 

 modernized hordes. But there was an additional incentive, for 

 throughout the early Tertiary there is evidence of climatic varia- 

 tion and of a very gradual cooling of the northern climate and a 

 consequent southward retreat of the higher plants and mammals 

 which occurred as a succession of migratory waves. In this way 

 there came, first, the least hardy like the insectivores and primates, 

 the latter especially depending so largely upon the tropical forests 

 for their sustenance that any change cither in extent or character 



