134 FROM FISH TO PHILOSOPHER 



period after birth. Also from the pantotherian stem, there 

 evolved the true or placental mammals, the Eutheria 

 ( = true beasts), in which the young are (with few ex- 

 ceptions, including monkeys, apes, and man), physio- 

 logically capable of taking care of themselves as soon as 

 they are weaned. In the Eutheria, the amniotic egg is 

 left within the womb to mature into a miniature adult, 

 thus supplying the developing embryo with an ideal 'ex- 

 ternal environment' until such a time as it can manu- 

 facture its own internal environment by foraging for 

 food and water for itself. 



The homeothermic mechanism is not perfected in all 

 mammals. In nearly all forms the regulation of body 

 temperature is poor in the newborn— a two-day-old 

 mouse, for example, is essentially a poikilothermic ani- 

 mal and does not suffer from the circumstance that its 

 body temperature drops considerably when the mother 

 leaves the nest. The platypus and the echidna, as well 

 as the opossum and other marsupials, the armadillo, bat, 

 v/oodchuck, hamster, sloth, and numerous others, are not 

 even strictly homeothermic as adults: during warm 

 weather they have a somewhat lower and more fluctuat- 

 ing body temperature than the advanced mammals, and 

 during cold weather they hibernate and the body tem- 

 perature falls to a level only slightly above that of the 

 environment. Hence they can more properly be called 

 Tieterothermic' (Many truly homeothermic animals, 

 such as bears and foxes, also hibernate in the winter, 

 but without marked reduction in body temperature; 

 they simply go to sleep and maintain their metabolism 

 at a slightly reduced level by burning their rich stores of 

 body fat.) Newly hatched birds also have poor body- 

 temperature regulation, but no adult heterothermic bird 

 is known, possibly because selection has left us only a 

 sample that is highly proficient in this respect. 



As homeothermy is not universal among existing mam- 

 mals, it is probably wise to look upon the Jurassic mam- 

 mals as at best heterothermic. Nevertheless, the tend- 



