lower animals, that the same classification and the same terms can be used
for both, as has been shewn by Isidore Geoffroy St.-Hilaire. (15. 'Hist.
Gen. et Part. des Anomalies de l'Organisation,' in three volumes, tom. i.
1832.) 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 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 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 (16.
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, 'De l'Influence des Milieux,'
etc. He lays much stress, in the case of plants, on the nature of the
soil.); but several are so important, that they must be treated at
considerable length.

THE DIRECT AND DEFINITE ACTION OF CHANGED CONDITIONS.

This is a most perplexing subject. It cannot be denied that changed
conditions 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