domesticated animals.
Not only does variability appear to be induced in man and the lower
animals by the same general causes, but in both the same parts of the
body are affected in a closely analogous manner. This has been proved
in such full detail by Godron and Quatrefages, that I need here only
refer to their works. (14. Godron, De l Espece, 1859, tom. ii. livre
3. Quatrefages, Unite de l Espece Humaine, 1861. Also Lectures on
Anthropology, given in the Revue des Cours Scientifiques, 1866-1868.)
Monstrosities, 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 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
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