MAMMALIA 335 



brain development. The habit of living in regions with a changeable 

 temperature would doubtless be associated with the development of 

 various temperature-regulating mechanisms that are to-day associated 

 with what we call warm-bloodedness. Probably the gradual develop- 

 ment of the homothermous condition paralleled the gradual separa- 

 tion of the right and left ventricles and the resultant complete sep- 

 aration of the arterial and venous blood. One of the consequences of 

 a higher temperature must have been a heightened nervous efficiency, 

 for it is well known that nervous tissues tend to develop more ef- 

 fectively at relatively high temperatures. Furthermore, the habit 

 of uterine gestation would increase the effectiveness of the higher 

 temperatures at the very time when the organism is most responsive; 

 for, as has been experimentally demonstrated, the early stages of 

 development are crucial in determining the character of the nervous 

 system. (In support of this view it may be said that the least highly 

 differentiated brain among modern mammals is that of the mono- 

 tremes in which there is a less effective temperature-regulating mech- 

 anism and in which a constant developmental temperature is im- 

 possible because the eggs are allowed to cool periodically while the 

 mother is absent in search of food. The most highly differentiated 

 brain, moreover, is found in Man, who has an exceptionally prolonged 

 period of uterine gestation, and who has learned the uses of clothing 

 and artificial heat as aids in maintaining a constant high body tem- 

 perature, especially in the young infant prior to the development of 

 its homothermic mechanism; for the human infant is for some time 

 after birth practically cold-blooded in the sense that it is unable to 

 maintain a constant temperature.) 



Time, Place and Environment of the Pro-mammals. — ^A knowl- 

 edge of the period when the pro-mammals lived should give a clew as 

 to the probable causes of the development of mammalian characters 

 in some reptiUan group. The place of origin of the first mammalian 

 experiments appears to have been South Africa, and the time early 

 Permian. The eminent palaeontologist and palseogeographer, Schu- 

 chert,says: "The evidence is now unmistakable that early in Permian 

 times all of the lands of the southern hemisphere were under the in- 

 fluence of a glacial climate as severe as the polar one of recent times, 

 and that, like the latter, the Permian one also had warmer interglacial 

 periods, for coal beds occur associated with glacial deposits in Aus- 

 traUa, South Africa, and Brazil." Now the cynodonts, which we 



