30 The Descent of Man. Part i 



Quatrefages, that I need here only refer to their works. 14 Mon- 

 strosities, which graduate into slight variations, are likewise so 

 similar in man and the lower animals, that the same classification 

 and the same terms can be used for both, as has been shewn by- 

 Isidore Geoffroy St.-Hilaire. 25 In my work on the variation of 

 domestic animals, I have attempted to arrange in a rude fashion 

 the laws of variation under ihe following heads : — The direct 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 oi 

 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 ; 18 

 but several are so important, that they must be treated at con- 

 siderable length. 



The direct and definite action of changed conditions. — This is a 

 most perplexing subject. It cannot be denied that changed con- 

 ditions 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 favour of this con- 

 clusion ; 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, by which the whole organisation is rend- 

 ered 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 



14 Godron, 'De l'Espece,' 1859, 16 I have fully discussed these 

 torn. ii. livre 3. Quatrefages, ' Unite laws in my ' Variation of Animals 

 de l'Espece Humaine,' 1861. Also and Plants under Domestication.' 

 Lectures on Anthropology, given in vol. ii. chap. xxii. and xxiii. M. J. 

 the ' Revue des Cours Scientiriques,' P. Durand has lately (1868) pub- 

 1866-1868. lished a valuable essay ' De l'ln- 



15 ' Hist. Gen. et Part, des Ano- fluence des Milieux,' &c. He lays 

 nialies de ^Organisation,' in tnree much stress, in the case of plants, on 

 rolcmes, torn, i, 1832. the nature of the soil. 



