342 ELEMENTARY BIOLOGY 



tapeworm, although perhaps not so serious, is probably more 

 common. Among insects are many related to the wasp and 

 the bee that lay their eggs in the bodies of caterpillars ; when 

 the young hatch out, they begin to feed on the caterpillar (see 

 5, Fig. 115). Among the backboned animals, certain fishes 

 will attach themselves to the bodies of other fishes and suck 

 the blood from their victims. 



Most vertebrates get their food either by killing plants or 

 other animals or by taking dead matter (that is, plant or ani- 

 mal remains) of one kind or another ; in other words, there 

 are very few vertebrate parasites. 



The idea of parasitism extends beyond the means of getting food. 

 The European cuckoo will lay her eggs in the nests of strange birds, 

 thus getting from other organisms at least two direct benefits the 

 work of building a shelter for the young and the work of keeping 

 the eggs warm during incubation; there is also the feeding of the 

 young through the work of the strange foster mother. This is a 

 case of getting services from another organism, without giving any- 

 thing in return. It is in this sense that we use the word parasitism 

 in connection with higher animals, and especially in connection with 

 human affairs. 



From the viewpoint of the unwilling hosts to the unbidden 

 guests, parasitism is an obstacle to life ; and every species of 

 living things is exposed to a number of such parasitic enemies. 

 To be able to protect itself against parasites is one of the 

 conditions necessary for maintaining life. 



397. The competitive relation. If all the offspring of any 

 plant or animal should reach maturity and reproduce the usual 

 number of young, and if this were continued for several gen- 

 erations, the earth would not be able to hold the resulting 

 population. 1 



1 A conger eel is said to lay 1 5,000,000 eggs in a year. If each of these 

 eggs hatched and reached maturity, and if each of these individuals repro- 

 duced at the same rate as the parents, the ocean would soon be crowded with 

 conger eels. The same thing is true of all animals. 



