RELATION OF ANIMAL MOTION TO ANIMAL EVOLUTION. 353 



are simply due to excess or defect of nutrition following a redis- 

 tribution of force.* 



The most direct evidence in support of the view that motion 

 aifects structure directly is to be found in the well-known phe- 

 nomenon of the increase of the size and power of all organs by 

 use. This increase is limited in the adult animal by the general 

 fixity of all the organs, so that one of them cannot be developed 

 beyond a certain point without injury to others, or without ex- 

 hausting the source of supply of nutritive material or special force 

 derived from other organs. The syncope of the gymnast is an 

 illustration of the natural limitation to the development of the 

 muscular system which proceeds at the expense of the digestive 

 and circulatory. But effort and exertion may become a habit of 

 mind, v/hich, even if limited in its executive means, is probably in- 

 herited by offspring like all other mental traits. Such a quality 

 possessed by an infant or child doubtless tells on the growth of 

 its organs during their plastic stage, and produces structure by 

 growth which is impossible to the mature body.f And no one 

 knows as yet how far mental bias may affect the nutrition of the 

 parts of the infant in utero. Certain it is, that if use modifies 

 nutrition in the adult, it must have still greater influence in the 

 young ; and it is in the young that the changes which constitute 

 evolution necessarily appear. 



Change of structure during groAvth is accomplished either by 

 addition of parts ("acceleration") or by subtraction of parts 

 ("retardation"). 



Acceleration is produced either by multiplication of parts (as 

 cells or segments) already present ("homotopy "), or by the 

 transfer of parts (cells) from one part of the organism to the 

 other ("heterotopy "). Homotopy or repetition is the usual and 

 normal mode of acceleration ; it may proceed by an " exact repeti- 

 tion" of the parts already existing, as in the simjilest animals and 

 plants ; or the new parts may differ from the old, as in higher 

 animals, where the process is called "modified repetition." Where 

 new forms traverse in their growth all the stages in which they 

 previously existed, they necessarily present at each stage the char- 

 acters of those forms which have remained stationary in them, and 

 have not changed. This relation of " exact parallelism " is the 



* "Method of Creation," 18Y1, p. 23. 

 f In raan these changes are chiefly produced in the brain. 

 23 



