Chap. IV. MANNER OF DEVELOPMENT. 113 



duate into slight variations, are likewise so similar in 

 man and the lower animals, that the same classifica- 

 tion and the same terms can be used for both, as may 

 be seen in Isidore Geoffroy St.-Hilaire's great work. 14 

 This is a necessary consequence of the same laws of 

 change prevailing throughout the animal kingdom. 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 shewn 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 vari- 

 ability of multiple parts. Compensation of growth ; 

 but of this law I have found no good instances 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 ; 15 but several are so im- 

 portant for us, that they must be treated at consider- 

 able length. 



The direct and definite action of changed conditions. — 

 This is a most perplexing subject. It- cannot be denied 



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

 volumes, torn. i. 1832. 



15 I have fully discussed these laws in my 'Variation of Animals 

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

 P. Durand has lately (1868) published a valuable essay ' Do l'lnfluence 

 des Milieux, &c.' He lays much stress on the nature of the soil. 



VOL I. I 



