98 



THE RISE OF ANIMAL LIFE 



at the expense of another. A predatory ani- 

 mal feeds upon another by eating its entire 

 body, frequently at one sitting. A parasite 

 might just as surely destroy the host, but 

 it does so in an entirely different manner. 

 A cat kills and eats a mouse; the cat is liv- 

 ing at the expense of the mouse. The cat 

 has a tapeworm inside its intestine; the 

 tapeworm is living at the expense of the 

 cat. The first case is predation, the second, 

 parasitism. "The difference between a pred- 

 ator and a parasite is simply the difference 

 between living upon capital and income, 

 between the burglar and the black-mailer. 

 The general results are the same although 

 the methods employed are different." (El- 

 ton) 



Parasitism probably arose shortly after 

 life originated on the earth. Some animals 

 soon found that they could live to advan- 

 tage either in or on the body of another. 

 Perhaps at first the relationship was a per- 

 fectly harmless one, something like com- 

 mensalism, but as the association persisted 

 the parasite became more and more de- 

 pendent on the host for its existence. It 

 modified its body both morphologically and 

 physiologically in accordance with its para- 

 sitic habit. In earlier relationships the para- 

 site probably clung to the outside of its 

 host, later going into the shallow cavities 

 such as the mouth and cloaca. Some, of 

 course, were inadvertently swallowed with 

 the food and these after a time became 

 adapted to life in the gut where all of their 

 food requirements were provided for. Oth- 

 ers that learned to live in the bodies of 

 insects such as mosquitoes, also learned to 

 thrive in the blood of vertebrates because 

 they were dumped into that environment 

 every time the mosquito feasted on a blood 

 meal. 



Through similar food chains parasites 

 must have learned to live first in one host, 

 then in another, and sometimes in a third, 

 all in sequence, in order to complete their 

 life cycle. This arrangement had the advan- 

 tage of spreading the species but it also had 



the serious disadvantage of depending not 

 only on three hosts but also on the necessity 

 that the hosts be sequentially arranged in 

 time and space. Should any one of the 

 hosts die out, the parasite would likewise 

 be destroyed because it could not com- 

 plete its cycle. In fact, this is a most effec- 

 tive way to control certain dangerous para- 

 sites. Killing mosquitoes in order to control 

 malaria is a familiar example. 



Parasites have been so intimately tied to 

 their hosts for so many millions of years 

 that ecologists today can often trace the 

 history of certain species by comparing 

 their parasites. In the subsequent chapters 

 we shall study several different kinds of 

 parasites as they occur in the various ani- 

 mal groups. 



Between animals in communities 



Up to this point in our discussion, rela- 

 tionships between individual animals have 

 been considered. In addition to this special 

 type of association, animals usually live in 

 a much larger community in which a great 

 many plants and animals are influenced by 

 each other, in some instances more inti- 

 mately than in others. 



Some biologists consider an entire plant- 

 and-animal community as a unit. Such a 

 "superorganism," that is, an organism in 

 which "the whole is greater than the sum 

 of its parts," is referred to as a biome. Just 

 how far one can draw a parallel between 

 a community of organisms and an individ- 

 ual is a questionable. It begins to smack of 

 anthropomorphism, which can be helpful 

 in certain circumstances, ridiculous in oth- 

 ers. Certainly it is well established that all 

 life in a community is tightly woven to- 

 gether, like the fibers of a spider's web, so 

 that no part can be disturbed without dis- 

 rupting the pattern. However, to compare 

 the community arrangement to the intri- 

 cate interrelationship of parts of the human 

 body, for example, is stretching the point 

 beyond recognition. 



Certain organisms live in specific habi- 



