4 PRACTICAL ZOOLOGY 



fever in man and Texas fever in cattle. These internal parasites 

 live within the bodies of the particular animals upon which they 

 prey, and must be adapted to the conditions there; for example, 

 the tapeworm and the hookworm amid the digestive juices in 

 the alimentary canal of man, and the malarial parasite in the 

 blood of man. Other parasites are said to be external, since they 

 do not penetrate into the body, but simply ride about on their 

 victim. Human beings are sometimes infested with external as 

 well as internal parasites; we need only mention the louse and 

 the flea. Even parasites are sometimes attacked by other 

 parasites, thus establishing the truth of the following lines: — 



" Great fleas have little fleas 

 Upon their backs to bite 'em, 

 And little fleas have lesser fleas, 

 And so ad infinitum." 



The relations between animals and their surroundings are 

 often very complex. Living creatures must not only be able to 

 cope with the state of temperature, moisture, and other physical 

 conditions of their habitats, but must also maintain more or less 

 complex relations with plants, other animals of the same kind, 

 and animals different from themselves. There is always a 

 struggle for existence among the lower animals, just as there is 

 among human beings who work so strenuously for homes and 

 power. In this struggle for existence the weak usually succumb 

 and as a result the strength of the race is maintained. One 

 curious fact is that other animals may depend upon, as well as 

 struggle with, one another. This may best be illustrated by 

 Charles Darwin's story of the field mice and humble bees. 

 Darwin found " that the visits of bees are necessary for the 

 fertilization of some kinds of clover; for instance, twenty 

 heads of Dutch clover yielded 2290 seeds, but twenty 

 other heads, protected from bees, produced not one." . . . 

 " Humble bees alone visit the red clover, as other bees cannot 

 reach the nectar, — hence we may infer as highly probable, 

 that, if the whole genus of humble bees became extinct or very 



