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 4 Hist. Gen. et Part, cles Anomalies de V Organisation/ in three 
Tolumes, tom. 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 4 De l’lnfluence 
des Milieux, &c.’ He lays much stress on the nature of the soil. 
VOL. I. I 
