34 THE DESCENT OF MAN. 



all or nearly all the individuals of the same species, varying 

 in the same manner under the same circumstances. The 

 effects of the long-continued use or disuse of parts. The 

 cohesion of homologous parts. The variability of multiple 

 [)arts. Compensation of growth ; but of this law I have 

 found no good instance in the case of man. The effects of 

 the mechanical pressure of one part on another; as of the 

 pelvis on the cranium of the infant in the womb. Arrests 

 of development, leading to the diminution or suppression 

 of parts. The reappearance of long-lost characters through 

 reversion. And lastly, correlated variation. And these 

 so-called laws apply equally to man and the lower animals ; 

 and most of them even to plants. It would be superfluous 

 here to discuss all of them; * but several are so important, 

 that they must be treated at considerable length. 



Tlie Direct and Definite Action of Changed Conditions. 

 This is a most perplexing subject. It cannot be denied 

 that changed conditions produce some, and occasionally a 

 considerable effect, on organisms of all kinds; and it seems 

 at first probable that if sufficient time were allowed this 

 would be the invariable result. But I have failed to obtain 

 clear evidence in favor of this conclusion; and valid reasons 

 may be urged on the other side, at least as far as the in- 

 numerable structures are concerned which are adapted for 

 special ends. There can, however, be no doubt that 

 changed conditions induce an almost indefinite amount of 

 fluctuating variability, by which the whole organization is 

 rendered in some degree plastic. 



In the United States, above 1,000,000 soldiers, who 

 served in the late war, were measured, and the States in 

 which they were born and reared were recorded. \ From 

 this astonishing number of observations it is proved tluit 

 local influences of some kind act directly on stature; and 

 we further learn that ^^ the State where the physical growth 

 has in great measure taken place, and the State of birth, 



* I have fully discussed these laws in my "Variation of Animals 

 and Plants under Domestication," vol. ii, cliap. xxii and xxiii. M. J, 

 P. Durand has lately (1868) published a valuable essay " De I'lnflu- 

 ence des Milieux," etc. He lays much stress, in the case of plants, 

 on the nature of the soil. 



f " Investigations in Military and Anthrop. Statistics," etc., 1869, 

 by B. A. Gould, pp. 93, 107, 126, 131. 134, 



