72 EVOLUTION 



The pre-Cambrian ocean was now swarming with 

 life. Its floors were dotted with sponges and corals and 

 hydrozoa dragging down unwary swimmers, and the 

 free-swimming animals had increased sufficiently to 

 initiate the struggle for life. In this struggle the animals 

 that chanced to have more evenly-balanced bodies, with 

 the sensitive cells located at the front, were best fitted 

 to survive. Rough sense-organs were developed in the 

 head. A pair of depressions in the skin lined with 

 pigment cells (to arrest and so better feel the light) 

 represent the first eyes: we find them still in some 

 lowly animals. Another couple of sensitive pits were the 

 first nose. A third depression in the skin with a sort of 

 stone rolling on a sensitive bed gave the animal some sense 

 of equilibrium and direction, and was destined to become 

 the ear of the land animal. A couple of rough channels 

 for carrying off waste matter formed the first kidney 

 system. The digested fluid food began to make definite 

 channels ("blood-vessels") in its course through the 

 body. The stomach began to protect its entrance with 

 a stout gullet. All these structures are really found 

 to-day in animals that linger at the lower levels of 

 development. 



On these general lines we conceive the further evolu- 

 tion of the animal body the formation of a definite 

 head and tail and long bilateral body, sweeping through 

 the water, like a Roman galley, by means of the rows of 

 cilia on its flanks. Thousands of different types of this 

 animal would be developed, and would offer various 

 starting-points for the evolution of the higher animals. 

 But I must pass very briefly over the rise of the higher 

 invertebrate classes, which is very obscure, and come to 

 the easier story of the evolution of the fish, the reptile, 

 the bird, and the mammal. 



If you examine in a museum a case of the earliest 



