Viviparity naturally finds its best illustrations among terrestrial 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 oviparous; 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 ante-natal life and an intimate partnership between the mother and the unborn young. Sec. 4 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 sea--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. [Illustration: THE HOATZIN INHABITS BRITISH GUIANA The newly hatched bird has claws on its thumb and first finger and so is enabled to climb on the branches of trees with great dexterity until such time as the wings are strong enough to sustain it in flight.] [Illustration: _Photograph, from the British Museum (Natural History), of a drawing by Mr. E. Wilson._ PERIPATUS A widely distributed old-fashioned type of animal, somewhat like a permanent caterpillar. It has affinities both with worms and with insects. It has a velvety skin, minute diamond-like eyes, and short stump-like legs. A defenceless, weaponless animal, it comes out at night, and is said to capture small insects by squirting jets of slime from its mouth.] [Illustration: _Photo: W. S. Berridge, F.Z.S._ ROCK KANGAROO CARRYING ITS YOUNG IN A POUCH The young are born so helpless that they cannot even suck. The mother places them in the external pouch, and fitting their mouths on the teats injects the milk. After a time the young ones go out and in as they please.] It is not an easy haunt of life, but none the worse for that, and it is tenanted to-day by representatives of practically every class of animals from infusorians to seashore birds and mammals. The Cradle of the Open Sea 2. The open-sea or pelagic haunt includes all the brightly illumined surface waters beyond the shallow water of the shore area.