SALPA IN RELATION TO EVOLUTION OF LIFE. 131 



animals where plant life is represented only by a few calcareous 

 algae, so strange in shape and texture that they are much less 

 plant-like than the true animals. 



The scarcity of vegetation becomes still more noticeable when 

 we study the ocean as a whole. 



On land, herbivorous animals are always much more abundant 

 and prolific than the carnivora, as they must be to keep up the 

 supply of food. Insectivorous birds are very abundant, but they 

 are not numerous enough to keep the plant-eating molluscs and 

 insects in check, and the devastation which is caused every year 

 by the armies of grasshoppers and locusts and herbivorous beetles 

 and by other less conspicuous insects, shows that their natural 

 enemies are not numerous enough to overtax their productive power. 



The birds which feed upon grain and seeds and fruit are very 

 abundant indeed, and they sometimes gather at their breeding 

 grounds, or places of assembly, in innumerable multitudes, but 

 the hawks and owls which prey upon them are never numerous. 



The small rodents, such as tlie rats, mice, squirrels, and rabbits, 

 are the most abundant and prolific of animals ; but the small 

 carnivora are so rare that their very existence is known to few 

 except naturalists and trappers. 



The homes of the wild sheep and goats, deer, antelopes, cattle and 

 horses support these large mammalia in incredible numbers, but 

 their carnivorous enemies are never abundant. It is clear that if 

 the destruction of the plant-eaters exceed their productive power, 

 both herbivora and carnivora would disappear, and terrestrial life 

 would come to an end. 



The animal life of the ocean shows a most remarkable difference, 

 for marine animals are almost exclusively carnivorous. 



The birds which live upon the ocean, the terns, gulls, petrels, 

 divers, cormorants, tropic birds and albatrosses, are very numerous 

 indeed ; so numerous that in many parts of the ocean some are 

 always visible in calm weather around the vessel, wherever it may 

 be. The only parallel to the pigeon-roosts and rookeries of the 

 land is found in the dense clouds of sea-birds around their breed- 

 ing places, but these sea-birds are all carnivorous ; most of them 

 are fishers, and others, such as the petrels, scoop up the copepods 

 and pteropods from the surface. Even the birds of the sea-shore 



