species are more variable than others, but that no species subjected to the
experiment persistently refuses to vary; and that, when it has once begun
to vary, its varieties are not the less but the more subject to variation.
"No case is on record of a variable being ceasing to be variable under
cultivation." It is fair to conclude, from the observation of plants and
animals in a wild as well as domesticated state, that the tendency to vary
is general, and even universal. Mr. Darwin does "not believe that
variability is an inherent and necessary contingency, under all
circumstances, with all organic beings, as some authors have thought." No
one supposes variation could occur under all circumstances; but the facts
on the whole imply a universal tendency, ready to be manifested under
favorable circumstances. In reply to the assumption that man has chosen for
domestication animals and plants having an extraordinary inherent tendency
to vary, and likewise to withstand diverse climates, it is asked:


"How could a savage possibly know, when he first tamed an animal, whether
it would vary in succeeding generations and whether it would endure other
climates? Has the little variability of the ass or Guinea-fowl, or the
small power of endurance of warmth by the reindeer, or of cold by the
common camel, prevented their domestication? I cannot doubt that if other
animals and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a state
of nature, and could be made to breed for an equal number of generations
under domestication, they would vary on an average as largely as the parent
species of our existing domesticated productions have varied."


As to amount of variation, there is the common remark of naturalists that
the varieties of domesticated plants or animals often differ more widely