456 The Outline of Science 



it a chance it may nip you savagely." But the question is inevi- 

 table: "Of what service is the ruse?" Would the carnivore or 

 the bird of prey that liked opossum flesh dogs won't touch it 

 care whether the creature is dead or pretending to be dead? 

 Mr. Ingersoll's ingenious suggestion is that playing 'possum is 

 an instinct that arose in the geological Middle Ages in relation 

 to the dull-witted big reptiles as a rule, land reptiles do not 

 feed on carrion and that it persists nowadays as an anachronism 

 in circumstances where it is oftener fatal than protective. 



3 



The Placental Mammals 



The third grade of modern mammals includes the carnivores, 

 the hoofed animals, the monkeys, and so on to all of which the 

 term "placental" is applied. In adaptation to the difficulties of 

 terrestrial life, there has been an evolution of viviparous arrange- 

 ments. The Monotremes, as we have seen, lay eggs ; the Marsu- 

 pials bring forth their young prematurely; the Placentals have 

 established a more or less prolonged ante-natal partnership be- 

 tween the mother and the unborn young. The linking structure 

 between the two is the placenta, which brings some of the blood- 

 vessels of the unborn young (or foetus) into close contact, 

 although not union, with the blood-vessels in the wall of the 

 mother's womb (or uterus). No solid particle, unless it be a 

 living microbe or a wandering white blood-corpuscle, can pass 

 from the mother to the offspring, but there is a transfusion of 

 fluid and gaseous material between the two partners. What does 

 the offspring get from its mother? Dissolved nutritive material, 

 oxygen, water, salts, and some subtle chemical messengers called 

 "hormones." What does the offspring give to the mother? 

 Dissolved waste-materials, carbon-dioxide, watery fluid, and 

 again some "hormones." The mother gives much and gets little; 

 but it seems justifiable to say that the internal secretions or hor- 

 mones contributed by the unborn offspring to the mother assist 



