406 METAPHYSICAL EVOLUTION. 



velopment of variation, and then, after securing the survival of 

 the best, wait again for the best to project its own variations for 

 selection. In the question as to whether the latter are any better 

 or worse than the characters of the parent, natural selection in no 

 wise concerns itself." 



In seeking for the causes of the origin of variation, the follow- 

 ing hypotliesis was proposed : 



" 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 econ- 

 omy 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 

 dentine within the pulp-cavity to meet the encroachments of at- 

 trition. The hands of a race of laborers are always larger than 

 those of men of other pursuits. Pathology furnishes us with a 

 host of hypertrophies, exostoses, etc., produced by excessive use, 

 or necessity 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 th^ 

 locality of new repetitions of parts already existing, and deter- 

 mines an increase of growth-force at the same time, by the in- 

 crease of food always accompanying increase of work done, ia 

 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 or organ used by contact with 

 the objects of its use. This would seem to be the remote ca^^se 

 of the deposit of dentine in the used tootli ; in the thickening 

 epidermis of the hand of the laborer ; in the wandering of the 

 lymph-cells to the scarified cornea of the frog in Cohnheim's ex- 

 periment. 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 



