MANNER OP DEVELOPMENT. 29 



same parts of the body are affected In a closely analogous man- 

 ner. This, has been proved in such full detail by Godron and 

 Quatrefages, that I need here only refer to their works." 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 shown by 

 Isidore Geoffrey 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 direct and 

 definite action of changed conditions, as exhibited by all or nearly 

 all the individuals of the same species, varying in the same man- 

 ner under the same circumstances. The effects of the long-con- 

 tinued 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 cmidltionf!. — 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 suf- 

 ficient time were allowed this would be the invariable result. 

 But I have failed to obtain clear evidence in favor 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 organization is ren- 

 dered in some degree plastic. 



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



"Godron, 'De I'Bspece,' 1859, tom. ii. livre 3. Quatrefages, 'Unite de 

 I'Espece Humaine,' 1861. Also Lectures on Anthropology, given in the 

 'Revue des Cours Soientiflques' 1866-1868. 



" 'Hist. Gen. et Part, des Anomalies de TOrganisation' in three vol- 

 umes, torn. i. 1832. 



1° I have fully discussed these laws in my 'Variation of Animals and 

 Plants under Domestication,' vol. li. chap. xxii. and xxiii. M. J. P. 

 Durand has lately (1868) published a valuable essay 'De I'lnfluence des 

 Milieux,' &c. He lays much stress, in the case of plants, on the nature 

 of the soil. 



