284 THE ORIGIN OF THE OLDEST FOSSILS, ETC. 



acquired the habit of resting on the bottom soon began to multi- 

 ply faster than their swimming allies ; and their asexually produced 

 progeny, remaining for a longer time attached to and nourished by 

 the parent stock, were much more favourably placed for rapid 

 growth. As the animals of the bottom live on a surface, or at 

 least a thin stratum, while swimming animals are distributed 

 through solid space, the rapid multiplication of bottom animals 

 must soon have led to crowding and to competition, and it quickly 

 became harder and harder for new forms from the open water to 

 force themselves in among the old ones, and colonisation soon 

 came to an end. 



The great antiquity of all the types of structure which are 

 represented among modern animals is therefore what we should 

 expect, for after the foundation of the fauna of the bottom was 

 laid it became, and has ever since remained, difficult for new forms 

 to establish themselves. 



Most of our knowledge of the sea bottom is from three 

 sources : from dredgings and other explorations ; from rocks which 

 were formed beyond the immediate influence of continents ; and 

 from the patches of the bottom fauna which have gradually been 

 brought near its surface by the growth of coral reefs ; and from all 

 these sources we have testimony to the density of the crowd of 

 animals on favourable spots. Deep-sea explorations give only the 

 most scanty basis for a picture of the sea bottom, but they show 

 that animal life may thrive with the dense luxuriance of tropical 

 vegetation, and Sir Wyville Thomson says he once brought up at 

 one time on a tangle which was fastened to a dredge over twenty 

 thousand specimens of a single species of sea urchin. The number 

 of remains of palaeozoic crinoids and brachiopods and trilobites 

 which are crowded into a single slab of fine-grained limestone is 

 most astounding, and it testifies most vividly and forcibly to the 

 wealth of life on the old sea-floor. 



No description can convey any adequate conception of the 

 boundless luxuriance of a coral island, but nothing else gives such 

 a vivid picture of the capacity of the sea-floor for supporting life. 

 Marine plants are not abundant on coral islands, and the animals 

 depend either directly or indirectly upon the pelagic food-supply, 

 so that their life is the same in this respect as that of animals in 

 the deep sea far from land. 



