LAWS OF ORGANIC DEVELOPMENT. 603 
the laborer are always larger than those of men of other pur- 
suits. Pathology furnishes us with a host of hypertrophies, exos- 
toses, etc., produced by excessive use, or necessity for increased 
means of performing excessive work. The tendency, then, in- 
duced by use in the parent is to add segments or cells to the or- 
gan 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 always accompany- 
ing 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 appear- 
ance 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 or organ used by contact 
with the objects of its use. This would seem to be the remote 
cause of the deposit of dentine in the used tooth, in the thick- 
ening epidermis of the hand of the laborer, in the wandering of 
the lymph-cells to the scarified cornea of the frog in Cohnheim’s 
experiment. You cannot rub the sclerotica of the eye without 
producing an expansion of the capillary arteries and corresponding 
increase in the amount of nutritive fluid. But the case may be 
different in the muscles and other organs (as the pigment cells of 
reptiles and fishes) which are under the control of the volition of 
the animal. Here, and in many other instances which might be 
cited, it cannot be asserted that the nutrition of use is not under 
the direct control of the will through the mediation of nerve force. 
Therefore I am disposed to believe that growth force may be, b 
the volition of the animal, as readily determined to a locality where 
an executive organ does not exist, as to the first segment or cell 
of such an organ already commenced, and that therefore effort is 
in the order of time, the first factor in acceleration. 
Effort and use have, however, very various stimuli to their ex- 
ertion. 
Use of a part by an animal is either compulsory or optional. 
In either case the use may be followed by an increase of nutrition 
under the influence of reflex force or of direct volition. 
A compulsory use would naturally occur in new situations which 
take place apart from the control of the animal, where no alterna- 
tives are presented. Such a case would arise in a submergence 
of land where land animals might be imprisoned on an island or 
