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 
