ADAPTATION. 103 



portant and fundamental biological law, viz., that the living 

 organism must ~be adapted to its environment, or, in other words, 

 that a certain harmony between organism and environment is 

 essential to the continuance of life, and any influence which tends 

 to disturb or destroy this harmony tends to disturb or destroy 

 life. Thus the aerial part of Pteris must be fitted to make ex- 

 changes with, and maintain its life in, the atmosphere, while the 

 underground part must be similarly "adapted " to the soil in 

 which it lives and with which it has to deal. 



At the same time it appears that the general environment of the plant 

 considered as a whole falls naturally into secondary or minor environments, 

 to which special portions of the plant must be adapted. Hence arises the 

 differentiation of parts fitted to cope with the various minor environments. 

 This necessity extends to every organ and tissue of the plant, each of which 

 has its peculiar function corresponding to its special environment. 



The aerial part of Pteris displays admirable adaptation to, or 

 harmony with, the environment in its stalk, which rises to a 

 point of vantage for procuring air and light, and in its broadly 

 spreading top, which is covered by a skin, tough and impervious to 

 prevent undue evaporation and consequent desiccation, yet trans- 

 lucent, to allow the sun's rays to reach the starch-making tissue 

 within. The rhizome also, with its pointed terminal buds, its 

 elongated roots, armed with boring tips, and its thick, fleshy 

 parenchyma for the storage of food, is well adapted to, and in 

 harmony with, its own special environment. In order to realize 

 this, we have only to imagine the fern to be inverted, the aerial 

 portion being planted in the earth, and the underground portion 

 lifted into the air and exposed to the winds and sunshine. Under 

 these circumstances the want of adaptation of the parts to their 

 respective environments would speedily become apparent. Yet 

 different as these parts now are, they have lately sprung from the 

 same cell. More recently they were barely distinguishable in a 

 mass of tissue, part of which turned upwards into the air, while 

 another part turned downwards into the earth. But as develop- 

 ment went on, the aerial and underground parts were progres- 

 sively differentiated, thus becoming more and more perfectly 

 adapted to the peculiar conditions by which each is surrounded. 



Thus it appears that the harmony between every part of the 

 organism and its environment is brought about by a gradual 



