SUMMARY OP PHYSIOLOGICAL DEVELOPMENT. 387 



dent forces, come those between the upper and under sides of 

 leaves; which, as we have seen, vary in degree as the con- 

 trasts of forces vary in degree, and disappear where these con- 

 trasts disappear. Equally clear proof is furnished 

 by animals, that the original uniformity of surface lapses into 

 multiformity, in proportion as the actions of the environment 

 upon the surface become multiform. In a Worm, burrowing 

 through damp soil which acts equally on all its sides, or in a 

 Tcenia, uniformly bathed by the contents of the intestine it 

 inhabits, the parts of the integument do not appreciably 

 differ from one another; but in creatures not surrounded by 

 the same agencies, as those that crawl and those that have 

 their bodies partially inclosed, there are unlikenesses of in- 

 tegument corresponding to unlikenesses of the conditions. A 

 snail's foot has an under surface not uniform with the 

 exposed surface of its body, and this again is not uniform 

 with the protected surface. Among articulate animals there 

 is usually a distinction between the ventral and the dorsal 

 aspects; and in those of the Arthropoda which subject their 

 anterior and posterior ends to different environing agencies, 

 as do the ant-lion and the hermit-crab, these become super- 

 ficially differentiated. Analogous general contrasts 

 occur among the Vertebrata. Fishes, though their outsides are 

 uniformly bathed by water, have their backs more exposed 

 to light than their bellies, and the two are commonly distinct 

 in colour. When it is not the back and belly which are thus 

 dissimilarly conditioned, but the sides, as in the Pleuronectidce, 

 then it is the sides which become contrasted; and there may 

 be significance in the fact that those abnormal individuals of 

 this order which revert to the ancestral undistorted type, and 

 swim vertically, have the two sides alike. In such higher 

 vertebrates as reptiles, we see repeated this differentiation 

 of the upper and under surfaces : especially in those of them 

 which, like snakes, expose these surfaces to the most diverse 

 actions. Even in birds and mammals which usually, by 

 raising the under surface considerably above the ground, 



