THE METHOD OF CEEATION OF OEGANIO FORMS. 195 



A. Oil the Location of Groivtli-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 econo- 

 my of type-growth among animals, and confine myself to the two 

 following. The effects of use are well known. We can not use 

 a muscle without increasing its bulk ; we can not long use the 

 teeth in mastication without inducing a renewed deposit of den- 

 tine within the pulp-cavity to meet the encroachments of attri- 

 tion. The hands of the laborer are always larger than those of 

 men of other pursuits. Pathology furnishes us with a host of hy- 

 pertrophies, exostoses, etc., produced by excessive use, or neces- 

 sity for increased means of performing 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 in- 

 crease of growth-force at the same time, by the increase of food 

 always 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 ap- 

 pears 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 thicken- 

 ing 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 can not rub the sclerotica of the eye without 

 producing an expansion of the capillary arteries and correspond- 

 ing 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 can not 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, througli the motive force 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 com- 



