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1878.| Relation of Animal Motion to Animal Evolution. 43 
uniform organization in this respect. But the successive modifi- 
cations which they present were, in past ages, most intimately 
connected with the progressive changes of the medium in which 
they lived, as to the volume of oxygen supplied for respiration, 
as compared with that of the vapor of water, carbonic acid gas, 
etc. But it must be here noted, in passing, that there are many 
structures in animals which have never been adaptive, but which 
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 
affects 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 
exhausting 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 diges- 
tive and circulatory. But effort and exertion may become a habit 
of mind, which even if limited in its executive means, is probably 
inherited 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.? 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 growth is accomplished either 
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 simplest animals and 
1 Method of Creation, 1871, p 
~ #,In man these changes are chet produced in the brain. 
~~ 
