230 THE FOUNDATIONS OF ZOOLOGY 



to become larger and to increase faster than pelagic animals. 

 Their sedentary life must have been favorable to both sexual and 

 asexual multiplication, and the tendency to increase by budding 

 must have been quickly rendered more active, and one of the first 

 results of life on the bottom must have been to promote the ten- 

 dency to form connected cormi, and to retain the connection 

 between the parent and the bud until the latter was able to obtain 

 its own food and to care for itself. The animals which first 

 acquired the habit of resting on the bottom soon began to multiply 

 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 favorably 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 colonization 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 favorable 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 paleozoic crinoids and brachiopods and trilo- 

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



