380 PHYSIOLOGICAL DEVELOPMENT. 



the universal difference between the imbedded ends and the 

 exposed ends of the higher plants. And then among the 

 less marked contrasts of surface answering to the less marked 

 contrasts in the incident forces, come those between the 

 upper and under sides of leaves ; which, as we have seen, 

 vary in degree as the contrasts of forces vary in degree, and 

 disappear where these contrasts disappear. Equally 



clear proof is furnished by animals, that the original uni- 

 formity 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 that 

 acts equalty on all its sides, or in a Tcema, 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 integument corresponding to unlike- 

 nesses 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 Articu lata 

 which subject their anterior and posterior ends to different 

 environing agencies, as do the Ant-lion and the Hermit-crab, 

 these become superficially differentiated. Ana- 



logous general contrasts occur among the Vertebrata. Fish, 

 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. Where it is not the 

 back and belly that are thus dissimilarly conditioned, but the 

 sides, as in the Pleuronectidce, then it is the sides that be- 

 come 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 sur- 



