Introduction 15 



which create currents in the water, carrying with 

 them the organisms needed for food. These fixed 

 animals, at some stage in their development, must 

 provide for dispersing themselves, and very com- 

 monly the larvae are actively motile. In other cases 

 it is the adult which is active, as with many insect 

 parasites, the larvae being incapable of moving away 

 from their host. The same necessity for the dis- 

 tribution of the species is seen in plants, whose 

 reproductive parts show many methods of dispersal. 

 In the lower plants, freely motile reproductive cells 

 are common. In the higher plants, the distribution 

 of the reproductive parts spores, seeds, etc. is 

 usually passive, but these structures are often modi- 

 fied so as to facilitate the distribution by special 

 means, such as dispersal by the wind of many 

 winged seeds and fruits, or the development of 

 hooks by which these adhere to animals and are 

 thus transported. 



The fixed position of plants involves a high de- 

 gree of adaptability, and they show a capacity for 

 growth and regeneration that can hardly be matched 

 in animals. The most highly specialized plants are 

 far less individualized than the majority of ani- 

 mals ; indeed it is not always easy to limit the indi- 

 vidual, as most plants may perhaps be looked upon 

 rather as a colony of united units than as a single 

 individual. Not only are the cells of plants less 

 various than those of animals, but there may be also 

 an almost unlimited repetition of similar organs. 



