THE DESCENT OR ORIGIN OF MAN 51 



be used for both, as has been shown by Isidore Geoffroy 

 St.-Hilaire. " In my work on the variation of domestic 

 animals, I have attempted to arrange in a rude fashion 

 the laws of variation under the following heads: The di- 

 rect and definite action of changed conditions, as exhibited 

 by 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 

 parts. 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. All 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. 



The direct and definite action of changed conditions. — This 

 is a most perple:^ing subject. It cannot be denied that 

 changed conditions produce some, and occasionally a con- 

 siderable effect, on organisms of all kinds ; and it seems at 

 first probable that if suificient 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 innumerable 

 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, 



" "Hist. Gen. et Part des Anomalies de 1' Organisation, "in three volumes, 

 torn. i. 1832. 



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

 under Domestication," vol. ii. chaps, xxii. and xxiii. M. J. P. Durand has 

 lately (1868) published a valuable essay, "De I'lnfluence des Milieux," etc. 

 He lays much stress, in the case of plants, on the nature of the soil. 



