98 THE BIOLOGY OF AN ANIMAL. 



biological law, viz. , that the living organistn must he adapted to 

 its environment^ or, in otlier words, that a certain harmony 

 between organism and envh'oninent is essential to the continu- 

 ance of life, and any influence which tends to disturb or destroy 

 this harmony tends to disturb or destroy life. The ada]3tation 

 may be either passive (structural) or active (functional). Struc- 

 tural adaptation is well illustrated, for instance, by the general 

 shape of the body, so well adaj^ted for burrowing through the 

 earth. Again, the delicate integument gives to the body the 

 flexibility demanded by the pecuKar mode of locomotion; it 

 affords at the same time a highly favorable respiratory surface — 

 a matter of no small importance to the worm in its badly-venti- 

 lated burrow ; and yet this delicate integument does not lead to 

 desiccation, because the animal lives alwavs in contact with moist 

 earth. The alimentary canal, long and complicated, is most 

 perfectly fitted for working over and extracting nutriment from 

 the earthy diet. The reproductive organs are a remarkable in- 

 stance of complex structural adaptation in an animal which on 

 the whole is of comparatively simple structure. 



Functional adaptation is perhaps best shown in the instinctive 

 actions or "habits" of the worm. Its nocturnal mode of life 

 (functional adaptation to light) and its ' ' timidity ' ' protect it 

 from heat, desiccation, from birds and other enemies. In win- 

 ter or in seasons of drought it burrows deep into the earth. 



A striking instance of adaptation is shown in the care which 

 is taken to insure the welfare of the embrvo worms. Minute, 



•y 7 



delicate, and helpless as they are, they develop in safety inside 

 the tough, leathery capsule (p. 78), floating in a milklike 

 liquid which is at once their cradle and their food. 



Origin of Adaptations. The development of the earthworm 

 shows that its whole complex bodily mechanism takes origin in a 

 single cell (p. 74), and that all the remarkable adaptations ex- 

 pressed in its structure and action are brought about by a gradual 

 process in the life-history of each individual worm. There is 

 reason to believe that this is tj^^ical of the ancestral history (de- 

 scent) of the species as a whole, and that adaptation has been 

 gradually acquired in the past. We know that environments 

 change, and that to a certain extent organisms change corre- 

 spondingly through functional adaptation, provided the change of 



