184 PRIMITIVE FORMS OF LIFE 



like the Reptiles and Birds. The small eggs of the present 

 Mammals produce a yolk-sac, an amnion, and an allantois 

 like those of the Birds, and these organs, which under 

 Mammalian conditions cannot be accounted for without 

 recourse to heredity, are utilized in a new manner. It is at their 

 expense, and with the more or less active assistance of the 

 womb, that a placenta is formed, by means of which the young 

 Mammal can obtain from the blood of its mother the nutritive 

 substances not found in the egg. When the organism suddenly 

 abandons the womb, as a result of the mother's accouchement, 

 these substances go on accumulating in the parent's blood, 

 where they are no longer required. It is then that the glands of 

 the skin intervene and eliminate them. Those on the ventral 

 surface, stimulated by the incessant friction or the suction of the 

 young over which the mother is lying in order to keep them 

 warm, grow larger and finally become the w//&-producing 

 mammas that for a long period will furnish the only food of the 

 newly-born. Probably these glands were first differentiated 

 as a consequence of the twin acts of laying and brooding in 

 the oviparous Mammals, whose young simply licked the walls 

 of the ventral cavity in which the egg was still incubated. 

 Subsequently these differentiated glands became localized in 

 the ventral pocket where the Marsupials carry their young. 

 In the placental Mammals they were eventually multiplied 

 in two symmetrical lines. Both their number and the position 

 occupied were gradually brought into relation with the size of 

 the litter and the habitual posture of the mother, in conformity 

 with Lamarck's principle that use and disuse influences the 

 development of the organs independently of natural selection. 

 Dogs, Cats, and Pigs, in spite of the differences separating them, 

 have numerous mammas, because of the equally large size of 

 their litters ; the Horse family, Ruminants, and Monkeys, 

 which produce but one or two at a time, have only two or four 

 mammas. In fleet Mammals, these mammas are concealed between 

 the posterior limbs in such a manner that the young are hidden 

 and protected, while being suckled, by the body of the mother, 

 who stands erect while it suckles. Animals which use 

 their arms in climbing, like the Monkeys and Sloths, or to 

 hook themselves on to resting-places, like the Bats, or which 

 raise the anterior region of the body out of the water in order 

 to feed their young, like the Sirenians, have the mammas 



