REPRODUCTIVE STATE ENDS DEVELOPMENT. 513 



his constituent structures, his nervous system, the successive character- 

 istics of an avertebrated animal, a fish, a turtle, a bird, a quadruped, a 

 quadrumanous animal, before he assumes the special human character- 

 istics. This is his cycle of life, and it is the same cycle in one case as 

 in another. 



But the moment that our view is thus enlarged, we see that it is not 

 the individual with which we should deal, for an individual we can scarce- 

 ly define, since he is continually differing from himself. It is with a cy- 

 cle of proceeding, or a course of operations that we are engaged, a series 

 of forms being the outward manifestation of the succeeding periods of 

 that cycle or course. 



An infant, though unlike both its parents in form, has run through a 

 career like that passed through by them both. Sexual differentiation, 

 which indeed is one of the last differentiations occurring, offers no excep- 

 tion to the truth of this remark. The similitude lies in the career, not 

 in the form taken at different epochs. 



The essential principle, then, is, not that an organism produces a like 

 organism, but it produces a germ which, being placed under The reproduct _ 

 similar circumstances, passes through a like career of devel- iv e state closes 

 opment, and at successive periods offers an orderly series of 

 forms. The career is commonly observed to close as soon as the capac- 

 ity for reproduction is assumed. Hence, in every organism, the assump- 

 tion of the reproductive state is the signal that the end of development 

 is at hand. 



It does not plainly appear what are the circumstances which give rise 

 to the assumption of this capacity ; nevertheless, it may take place at 

 any moment of the career. In the Volvox globator it occurs almost at 

 the close of the first stage, for the germ only reaches the condition de- 

 scribed hereafter as the mulberry mass when it becomes capable of re- 

 production ; but in man the developing organism has a long journey to 

 perform beyond this first step. Except in the condition here dwelt upon, 

 he differs in no respect from his humbler comrade at this point. The 

 tendency to a gliding off into the reproductive phase is in him repressed, 

 and therefore differentiation and development continue to go on. 



During the development of any new organism, the new parts uniformly 

 arise from the old ones ; they are not built from foreign mate- All the parts of 

 rials depositing themselves upon new centres, but are educed an . organism 

 by the unfolding, enlarging, and modeling of parts already 



common cen- 



existing. An organism is not developed as we enlarge a tral origin- 

 house, by building part to part, but it all expands from one common or 

 single centre. As the sphere of its expansion becomes greater, the op- 

 portunity arises for devoting different regions to different uses, and thus 

 offices which were confusedly intermingled become separated out, and, 



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