184 MORPHOLOGICAL DEVELOPMENT. 



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 move- 

 ments are so exceptional and unobtrusive in the one kin^ 

 dom, 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 morpho- 

 logical differentiation, becomes, among animals, the chief 

 cause of morphological differentiation. 



Booted animals or animals otherwise fixed, of course pre- 

 sent traits of structure nearest akin to those we have lately 

 been studying. The motions of parts in relation to one another 

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

 gation and mode of fixing, we are presented with morpho- 

 logical 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 reactions. 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 movement — 

 a radial symmetry. If, in addition to this habitual 



attitude of the ends, one surface of the body is always upper- 

 most 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 that this 



