94 Some Characteristics of Sea-shore Fauna, [Sess. 



respiration, of secretion and of digestion (or what corresponds 

 to these functions in the case of less highly organised forms), 

 must go on even when the organism is in a state of restful 

 inactivity, the reserve stores get consumed, and must therefore 

 be replenished if life is to be maintained. And the need for 

 restoking the furnace is much greater in the case of an active 

 animal, since muscular exertion and nervous strain imply loss 

 of vitality. Hence the natural craving for food — to keep up 

 vitality. Thus, to live animals must eat, and to eat they 

 must find food. And so the question of food-supply is prim- 

 arily the explanation of their varied distribution. 



In a general way it may be said that kindly mother-earth 

 is the source of all food, for the earth is covered with a coating 

 of vegetation whose constituents are the ingredients of air and 

 soil, and animals live largely on plant life. They find plants 

 clothing the earth; millions of diatoms in their tiny flinty 

 capsules float on the surface of the ocean ; the boundary 

 between land and sea — the entire coast-lines of the world — 

 is fringed with seaweeds. Plant life there is in abundance, 

 but not a few animals have discovered that there are other 

 diets as satisfying as vegetarian ones. Some eat mud, and 

 some eat sand (merely of course for the organic material found 

 in these), and while many are vegetarians, there are also many 

 carnivorous animals, and some of these are cannibals. There 

 are so many microscopic forms in a gallon of the purest sea- 

 water, that not a few creatures depend on that alone for their 

 subsistence. Animals seek to dwell where they can most 

 easily get the food that suits them, and many are the adapta- 

 tions of structure they possess which fit them to capture and 

 to assimilate the food on which they depend. 



To perpetuate the race, hardly less numerous are the 

 methods that nature employs. Sometimes the sexes are dis- 

 tinct, sometimes they are not ; reproduction may be by cellular 

 division or by budding ; the young may be the result of par- 

 thenogenesis ; there may be a wonderful life cycle, involving 

 alternation of generations, or, as in most of the higher animals, 

 the young may be the result of the mating of the adults of 

 different sexes. Each type of animal has its own distinctive 

 method of providing for the continuance of the race. 



Think of the keenness of the struggle for existence in 



