REPRODUCTION AND HEREDITY 281 



plant matter, and hatch their eggs by the heat produced in the 

 heaps by fermentation. In order to regulate the temperature in 

 the interior of the heap they actually build several ventilating 

 shafts. Other birds even understand how to make use of the 

 volcanic activity of their own countries, and bury their eggs in 

 the warm ashes of volcanoes. 



The highest stage of perfection in the care of the offspring is 

 reached by the mammals, among whom the parental care forms 

 such striking characteristics that the whole group has derived 

 from it its name. In most mammals the young pass their 

 entire embryonic development in the interior of the maternal 

 body, and are fed immediately from the blood-circulation of the 

 mother. They are born developed but helpless creatures, and 

 must be suckled for a considerable time with the secretions from 

 the maternal milk-glands until they are able to obtain their food 

 independently. Only the Monotremata of Australia lay eggs like 

 reptiles and birds. These eggs are either hatched in the nest, as 

 in the case of Ornithorhynchus paradoxus, or in a pouch on the 

 ventral surface of the mother, as in the case of Echidna aculeata. 



In the marsupials the young are born in a very incomplete, 

 helpless state. The Giant Kangaroo, for instance, bears barely 

 forty days, and produces a naked, blind embryo, the size of a 

 walnut, without extremities. Carefully the mother seizes it with 

 the lips, and places it into the pouch, where it at once commences 

 to suckle. After about seven months the embryonic development 

 is complete, and for the first time the young looks shyly out into 

 the world. Soon it leaves the pouch, first for a short time, but 

 gradually for longer periods. But even fairly large young 

 kangaroos dive, at the least sign of danger, with a bold jump into 

 the maternal pouch until they are finally forcibly prevented in the 

 interests of a new generation. 



I could mention many more attractive features of the life- 

 habits of other mammals, but time forbids, and I will, there- 

 fore, now turn to the formation of germ-cells. 



The problem of fertilization and reproduction has occupied 

 the human mind for many ages. Until the discovery of the 

 microscope this was therefore a region which the mind in vain 

 endeavoured to invade. All that was possible was tentatively to 

 build up hypotheses. As the act of impregnation itself was 



