82 The Outline of Science 



to face a modern world too severe for it, one of the methods of 

 meeting the environing difficulties is the retention of the offspring 

 for many months within the mother, so that it is born a fully- 

 formed creature. There are only a few offspring at a time, and, 

 although there are exceptional cases like the summer green-flies, 

 which are very prolific though viviparous, the general rule is that 

 vivi parity is associated with a very small family. The case 

 of flowering plants stands by itself, for although they illustrate 

 a kind of viviparity, the seed being embryos, an individual 

 plant may have a large number of flowers and therefore a huge 

 family. 



Viviparity naturally finds its best illustrations among ter- 

 restrial animals, where the risks to the young life are many, and 

 it finds its climax among mammals. 



Now it is an interesting fact that the three lowest mammals, 

 the Duckmole and two Spiny Ant-eaters, lay eggs, i.e. are ovipa- 

 rous; that the Marsupials, on the next grade, bring forth their 

 young, as it were, prematurely, and in most cases stow them away 

 in an external pouch ; while all the others the Placentals show 

 a more prolonged antenatal life and an intimate partnership 

 between the mother and the unborn young. 



There is another way of looking at the sublime process of 

 evolution. It has implied a mastery of all the possible haunts of 

 life ; it has been a progressive conquest of the environment. 



1. It is highly probable that living organisms found their 

 foothold in the stimulating conditions of the shore of the sca- 

 the shallow water, brightly illumined, seaweed-growing shelf 

 fringing the Continents. This littoral zone was a propitious, 

 environment where sea and fresh water, earth and air all meet, 

 where there is stimulating change, abundant oxygenation and a 

 copious supply of nutritive material in what the streams bring 

 down and in the rich seaweed vegetation. 



