Chap. IV.] MANNER OF DEVELOPMENT. 109 



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 ar- 

 range in a rude fashion the laws of variation under 

 the following heads: The direct and definite action of 

 changed conditions, as shown by all or nearly all the in- 

 dividuals 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 homol- 

 ogous parts. The variability of multiple parts. Com- 

 pensation of growth ; but of this law I have found no 

 good instances in the case of man. The effects of the me- 

 chanical pressure of one part on another ; as of the pelvis 

 on the cranium of the infant in the Avomb. Arrests of de- 

 velopment, 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 ani- 

 mals ; and most of them even to plants. It would be 

 superfluous here to discuss all of them ; '^ but several are 

 so important 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 

 that changed conditions produce some effect, and occa- 

 sionally 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 



^^ 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, 'Del'Influence des 

 Milieux,' etc. He lays much stress on the nature of the soil. 



