On account of the extreme variability of the sheep under culture, it is generally supposed that the innumerable domestic breeds have all been derived from the few wild species; but the whole question is involved in obscurity.

In fact, all the variability among domesticated Species is due to the fostering care, or, in its more extravagant freaks, to the fancies of man, and it has never been observed in the wild Species, where, on the contrary, everything shows the closest adherence to the distinct, well-defined, and invariable limits of the Species.

On account of the great variability in strength of different specimens of wood even from the same stick and appearing to be alike, it is important to eliminate as far as possible all extraneous factors liable to influence the results of the tests.