MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 167 



organism in relation to one another, or both. Though there 

 are plants, especially of the simpler kinds, which move, 

 and though a few of the simpler animals do not move ; yet 

 movements are so exceptional and unobtrusive in the one 

 kingdom, while they are so general and conspicuous in the 

 other, that the broad distinction commonly made is well 

 warranted. What, among plants, is an inappreciable cause 

 of morphological differentiation, becomes, among animals, the 

 chief cause of morphological differentiation. 



Animals that are rooted or otherwise fixed, of course present 

 traits of structure nearest akin to those we have been latelv 

 studying. The motions of parts in relation to one another 

 and to the environment, being governed by thp. mode of aggre- 

 gation and mode of fixing, we are presented with morphological 

 differentiations similar in their general characters to those of 

 plants, and showing us parallel kinds of symmetry under 

 parallel conditions. But animals which move from place 

 to place are subject to an additional class of actions and re- 

 actions. These actions and reactions affect them in various 

 ways according to their various modes of movement. Let us 

 glance at the several leading relations between shape and 

 motion which we may expect to find. 



If an organism advances through a homogeneous medium 

 with one end always foremost, that end, being exposed to 

 forces unlike those to which the other end is exposed, may 

 be expected to become unlike it ; and supposing this to be 

 the only constant contrast of conditions, we may expect an 

 equal distribution of the parts round the axis of move- 

 ment a radial symmetry. If in addition to this 

 habitual attitude of the ends, one surface of the body is 

 always uppermost and another always lowermost, there arise 

 between the top and bottom dissimilarities of conditions, 

 while the two sides remain similarly conditioned. Hence it 

 is inferable that such an organism will be divisible into 

 similar halves by a vertical plane passing through its axis of 

 motion will have a bilateral symmetry. We may presume 

 VOL. II. 8 



