BUTTERFLIES AND BEES 37 



not easy to decide at once which organs in one case are analo- 

 gous to organs in the other ; and as for homology, most people 

 never discover any at all between plants and animals. Many 

 plants have no special breathing organs that are strictly analo- 

 gous to our nostrils or to the spiracles of insects, for they may 

 absorb oxygen from the air at any part of their surface. ^lost 

 plants have no organs that are analogous to the mouth which 

 is present in most animals, for plants do not take food as we 

 do, but simpler materials w^hich they later work over into food. 

 Plants have no organs that are analogous to the heart, for the 

 circulation of material within the organism is in their case 

 brought about by a totally different process. Even their irri- 

 tability and their movements depend upon different structures. 



On the other hand, the growth of plants, like the growth of 

 animals, is a process that is carried on by every part of the 

 body, and not by a special organ. Indeed, we shall see presently 

 that all the fundamental functions are really carried on by all 

 parts of the body. The idea of division of labor, the idea of 

 homology, and the idea of analogy all correspond to real facts ; 

 but they do not apply to every detail of life. 



30. Being alive. Whatever organism we may be thinking 

 about, we shall always find that so long as it is alive it keeps on 

 doing certain things, and that it remains alive only so long as 

 it does these things, that is, so long as it (i) takes food and 

 assimilates it, (2) transforms the assimilated material so as to 

 get from it the energy for movement or for other processes, and 

 (3) responds more or less suitably to changes going on around it. 



Our own bodies and those of the other animals that we have 

 studied are alive because they show irritability, assimilation, 

 and contractility ; and any other animal and any plant is alive 

 because it is sensitive to changes, because it transforms ma- 

 terial from outside into its own body, and because it can bring 

 about movements or other changes that fit the conditions. 



Like the insects that we have studied, every plant and every 

 animal also originates from an egg or some other structure 

 that is analogous or homologous to an egg. 



