EVOLUTION 245 



which we see about us to-day. Thne is the background for the story of 

 life. 



The ancestry of man is a long one. It goes back beyond Neanderthal 

 man; beyond the ape, the early mammal, the fish; beyond whatever in- 

 vertebrate ancestor the fish had; back to some single-celled form in pre- 

 Paleozoic seas nearly two billion years ago. A long time for man to be at 

 school. Progress has been slow, for Nature is a severe school mistress. 

 Failure in her school means more than just waiting over a grade; the 

 organism is thrown on the discard. She insists that her lessons be so learned 

 that they become part and parcel of the organism, and she takes all the 

 time necessary to secure that end. 



Somewhere in the pre-Paleozoic sea existed single-celled ancestors of 

 man, possibly like the amoeba. It could eat, assimilate, breathe, move, 

 reproduce, all in very simple ways; but these were the essential functions 

 of life. It was perfectly adapted to its environment; so perfectly that some 

 of its descendants, still amoebae, are with us to-day. It is no newcomer 

 into that early ocean; it had been at school for tens, perhaps hundreds of 

 millions of years, and the lessons of that infantile grade had become a 

 part of its very structure and function. We came along perhaps a thou- 

 sand million years to the mid-Paleozoic, the Devonian, the age of fishes. 

 Our ancestor is now a fish, admirably adapted to its medium, doing the 

 same things the amoeba had been doing, but in a more elaborate way. And 

 there were those milHons of years of schooling between. Some of the 

 Devonian fish, certain ganoids, were ready for a higher grade. In these 

 the swim-bladder opened into the throat, and in extremis could be used 

 for gulping air, that is, breathing. The fins were stout, and again, in 

 extremis could be used for crude walking. Imagine these ganoids, caught 

 season after season in time of drought in muddy pools on the Devonian 

 flats, gulping air and floundering about, half in and half out of water. It 

 was hard schooling, but some graduated; the air bladder developed into 

 a lung, the fins into legs; and these ganoids were the ancestors of all the 

 higher vertebrates. In the monotonous uniformity of the sea, evolution 

 of the higher types of life would have been impossible; that required one 

 variety of the land surface. The coming to land of these Devonian fish 

 has been said to be "the most momentous step in the whole advance from 

 amoeba to man." 



Two hundred million years further down the line we are in the mid- 

 Mesozoic. Great reptiles rule the air, the sea and the land. It looks as if 

 their future were secure, that their reign would last forever. It did last 

 for more than a hundred million years. But the future lay not with the 

 reptilian giants, but with certain small primitive mammals, an offshoot of 

 the line of reptiles. They had been waiting their chance for some tens of 

 million years. At the beginning of the Cenozoic, Nature took them in hand 

 for training for the higher grades; sixty million years of schooling it was 



