EVOLUTION AND ITS CONSEQUENCES. 27 



ones have been produced by location of growth-force without the 

 influence of use, or by its subtraction, due to a disturbance of 

 equilibrium, consequent on the special location elsewhere. 



But we go back to the origin of the question in investigating 

 the action of growth-force undisturbed by the interference of lo- 

 cating influences. This is only to be ascertained by an examina- 

 tion of lower organisms in connection with the higher. A point 

 that first strikes the student of higher animals is, that after he has 

 proved the law of homology to be true, as regards different animals 

 when compared, he further finds that the parts of the same ani- 

 mal are also homologous, that is, right hand and left, front and 

 behind, bottom and top are also constructed on the same plan, so 

 as to be reduced to the same elemental parts. This is described 

 under the names of "anteroposterior" and "bilateral symmetry," 

 etc.* These laws apply to all parts of the organism, and, though 

 there appear to be exceptions, these are traceable to a common 

 symmetrical t}^e, the change wrought being one form of speciali- 

 zation. It is not surprising then that the lower we descend in the 

 scale of life the less marked is the departure from the entirely 

 symmetrical disposition of parts as regards the common center. 

 This is beautifully seen in the Radiata, etc., where the jelly-fish, 

 the star-fish, and the sea-urchin are familiar examples. 



But these are complex compared with forms below them. The 

 minute Polycystina construct shells of beautiful sculpture or 

 tracery ; but, though they present many forms, they may be all 

 referred as modifications, to the symmetrically-marked disk. Of 

 cellular animals, Trichodiscus represents a discoidal aggregate, 

 and Adinoplirys a globular mass of cells. Monas and its allies 

 consist of one or several cells in globular form. 



Lastly, in the first appearance of life in the most complex ani- 

 mal we have first a cell, which, then "segmenting," divides itself 

 so as to become a globular mass of identical cells, arranged about 

 the center. 



From the cellular animal upward the operation of addition of 

 parts is then one of repetition, and that centrifugally in the case 

 of bodies which are developed in all planes, or peripherally in those 

 which are in one plane. 



Thus the complication of the edges of the septa of the am- 

 monites is accounted for. First, the growing animal was supplied 



* See Prof. B. Wilder, in Proceed. Boston Society Nat. Hist. 



