602 LAWS OF ORGANIC DEVELOPMENT. 
The size of a part is then dependent on the amount of cell di- 
vision or growth force, which has given it origin, and the number 
of segments is due to the same cause. The whole question, then, 
of the creation of animal and vegetable types is reduced to one 
of the amount and location of growth force. 
Before discussing the influences which have increased and lo- 
cated growth force, it will be necessary to point out the mode in 
which these influences must necessarily have affected growth. | 
Acceleration is only possible during the period of growth in ani- 
mals, and during that time most of them are removed from the 
influence of physical or biological causes either through their 
hidden lives or incapacity for the energetic performance of life 
functions. These influences must, then, have operated on the 
parents, been rendered potential in their reproductive cells, and 
become energetic in the growing foetus of the next generation. 
However little we may understand this mysterious process, it is 
nevertheless a fact. Says Murphy, “ There is no act which may not 
become habitual, and there is no habit that may not be inherited.” 
Materialized, this may be rendered—there is no act which does 
not direct growth force, and therefore there is no determination 
of growth force which may not become habitual; there is, then, 
no habitual determination of growth force which may not be in- 
herited ; and of course in a growing foetus becomes at once ener- 
getic in the production of new structure in the direction inherited, 
which is acceleration. 7 
Ill. THE INFLUENCES DIRECTING GROWTH FORCE. 
Up to this point we have followed paths more or less distinctly 
traced in the field of nature. The positions taken appear to me 
either to have been demonstrated or to have a great balance of 
probability in their favor. In the closing part of these remarks I 
shall indulge in more of hypothesis than heretofore. 
What are the influences locating growth force? First, physical 
and chemical causes ; second, use ; third, effort. I leave the first, 
as not especially prominent in the economy of type growth 
among animals, and confine myself to the two following. The 
effects of use are well known. We cannot use a muscle without 
increasing its bulk; we cannot use the teeth in mastication 
without inducing a renewed deposit of dentine within the pulp- 
cavity to meet the encroachments of attrition, The hands of 
