Cope. ] 246 (Dee. 15, 
But if the forces converted into growth force are derived from without 
the animal organism, whence and what the agency by which the acceler- 
ation or retardation of the latter is inherited from the parent? <A few 
suggestions only on this head can be made in the fourth section. 
Ill. THE LAW OF USE AND EFFORT. 
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. Inthe closing part of this paper I shall indulge in more of hy- 
pothesis than heretofore. 
Since repetitive addition only produces identical results in archetypes, 
and each effort produces results more and more unlike its predecessor as 
structure becomes specialized; it. becomes important to examine into 
the influences which have originally modified the repetitive efforts suc- 
cessively, producing structures more or less different in detail in the sec- 
ond generation from those of the parents, in acceleration, or the reverse, 
in retardation. : 
Going further back, the question arises, why a simple exhibition of 
repetition (¢. g., cell division) should be converted into a complex or du- 
plicated repetition (¢. g., jointed ray). This it has already been stated, is 
one consequence of increased amount of the growth force. 
We then seck explanation of the main question, as to what determines 
the location of this additional or new growth force. (Div. A.) 
Lastly, why the total amount of this force should change in a given 
individual or part of an individual. (Div. B.) 
A. Ow tan Location or GrowTH FORCE. 
What are the influences locating growth force? The only efficient ones 
with which we are acquainted, are, first, physical and chemical causes; 
second, use; and I would add a third, viz: 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 long 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 the laborer are always larger than those of men of other pur- 
suits. Pathology furnishes us with a host of bypertrophies, exostoses, 
etc., produced by excessive use, or necessity for increased means of per- 
forming excessive work. The tendency, then, induced by use in the 
parent, is to add segments or cells to the organ used. Use thus determines 
the locality of new repetitions of parts already existing, and determines 
an increase of growth force at the same time, by the increase of food al- 
ways accompanying increase of work done, in every animal. 
But supposing there be no part or organ to use. Such must have been. 
the condition of every animal prior to the appearance of an additional 
digit or limb or other useful element. It appears to me that the cause of 
the determination of growth force is not merely the irritation of the part 
