12 ANIMAL STUDIES 



the composition of protoplasm may change somewhat under 

 certain circumstances. It certainly is not everywhere alike, 

 for that of one animal must differ from that of another, and 

 different parts, such as the liver and brain, of the same form 

 must be unlike. These differences, however, are minor 

 when compared to the resemblances, for, as we shall see, 

 this living substance, wherever it exists, carries on the pro- 

 cesses of waste, repair, growth, sensation, contraction, and 

 the reproduction of its kind. 



13. Animal functions. Animals in general lead active, 

 busy lives, collecting food, avoiding enemies, and producing 

 and caring for their young. While the activities of all 

 animals are directed to their own preservation and to the 

 multiplication of their kind, these processes are carried on 

 in the most diverse ways. The manner in which an organ 

 or an organism is made, and the method by which it does 

 its work, are mutually dependent one on the other. As 

 there is an enormous number of species of animals, each 

 differently constructed, there is, accordingly, a very great 

 variety of habits. As we shall see, the lower forms are 

 remarkably simple in their construction, and their mode of 

 existence is correspondingly simple. In the higher types 

 a much greater complexity exists, and their activities are 

 more varied and are characterized by a high degree of elabo- 

 ration. In every case, the animal, whether high or low, is 

 fitted for some particular haunt, where it may perform its 

 work in its own special way and may lead a successful life 

 of its own characteristic type. 



