Origin of Different Kinds of Adaptations 341 



first problem is to examine in what sense the form itself 

 may be looked upon as an adaptation to the surroundings. 



It is a well-recognized fact that the forms of many animals 

 appear to stand in a definite relation to the environment. 

 For instance, animals that move in definite directions in 

 relation to their structure have the anterior and the pos- 

 terior ends quite different, and it is evident that these ends 

 stand in quite different relations to surrounding objects ; 

 while, on the other hand, the two sides of the body which 

 are, as a rule, subjected to the same influences are nearly 

 exactly alike. The dorsal and the ventral surfaces of the 

 body are generally exposed to very different external condi- 

 tions, and are quite different in structure. 



The relation is so obvious in most cases that it might 

 lead one quite readily to conclude that the form of the ani- 

 mal had been moulded by its surroundings. Yet this first 

 impression probably gives an entirely wrong conception of 

 how such a relation has been acquired. Before we attempt to 

 discuss this question, let us examine some typical examples. 



A radial type of structure is often found in fixed forms, 

 and in some floating forms, like the jellyfish. In a fixed 

 form, a sea-anemone, for instance, the conditions around the 

 free end and the fixed end of the body are entirely different, 

 and we find that these two ends are also different. The 

 free end contains the special sense organs, the mouth, ten- 

 tacles, etc.; while the fixed end contains the organ for attach- 

 ment. It is evident that the free end is exposed to the same 

 conditions in all directions, and it may seem probable that 

 this will account for the radial symmetry of the anemone. 

 There are also a few free forms, the sea-urchin for instance, 

 that have a radial symmetry. Whether their ancestors were 

 fixed forms, for which there is some evidence, we do not 

 know definitely ; but, even if this is true, it does not affect 

 the main point, namely, that, although at present free to 

 move, the sea-urchin is radially symmetrical. But when we 



