2 88 GENERAL BIOLOGY 



Nature has set bounds to which all the Hving must con- 

 form themselves. This is seen not alone in externals of form, 

 l3Ut also in the very fundamentals of organization. Even 

 the types of animal symmetry correspond to evironment. 

 Of the three main types, spherical symmetry, like that of 

 volvox (symmetry about a point) prevails where uniform 

 conditions exist on all sides of an organism; radial symme- 

 try like that of hydra and most plants (symmetry about a line) 

 prevails when conditions are alike at the sides of the axis of 

 the body but differ at the two ends; and bilateral symmetry 

 like that of the higher animals (symmetry about a plane), 

 prevails when conditions are alike upon tAvo sides but differ- 

 ent above and below as well as before and behind, as they 

 must be in all organisms that travel over the surface of solids. 



On the other hand, there are phenomena of divergent 

 development, of the persistence of types through the 

 vicissitudes of all environmental changes, of grotesqueries of 

 form, and superfluities of structure and ornamentation, that 

 speak most strongly for the dominance of the inner forces of 

 life, and that negative or minimize external influences. 



But it is not wise to exclude the possible action of either 

 inward or outward forces in development when we know 

 that both are ever present. The sightless condition of the 

 fishes that live in the underground streams of caves in total 

 darkness has often been treated as though it were a case of 

 pure adaptation. But when we note that other fishes 

 belonging to the same family (Amblyopsid ae) have weak 

 eyes and incline to stay in the deeper shadows of the shores, 

 we see that a racial tendency toAvard this sort of develops 

 ment may have favored the adaptation. Nature may have 

 segregated the fishes best suited to cave life in the environ- 

 ment best suited to them, and then may have gone on per- 

 fecting the adaptation, either directly, or by perfecting the 

 tendency, or by both means concurrently. Inherent ten- 

 dencies and environmental influences are ever present, and 

 development can only be the resultant of their interaction. 



