It is unquestionable that the degrees of variation which have been arrived at in domestic animals have been obtained more or less readily in a moderate amount of time, but that further development in certain desired directions is in some a matter of extreme difficulty, and in others appears to be all but, if not quite, an impossibility.

It is also unquestionable that the degree of divergence which has been attained in one domestic species is no criterion of the amount of divergence which has been attained in another.

It is contended on the other side that we have no evidence of any limits to variation other than those imposed by physical conditions, such, _e.g._, as those which determine the greatest degree of speed possible to any animal (of a given size) moving over the earth's surface; also it is said that the differences in degree of change shown by different domestic animals depend in great measure upon the abundance or scarcity of individuals subjected to man's selection, together with the varying direction and amount of his attention in different cases; finally, it is said that the changes found in nature are within the limits to which the variation of domestic animals extends,--it being the case that when changes of a certain amount have occurred to a species under nature, it becomes _another species_, or sometimes _two or more other species_ by divergent variations, each of these species being able again to vary and diverge in any useful direction.
But the fact of the rapidly increasing difficulty found in producing by ever such careful selection, any further extreme in some charge already carried very far (such as the tail of the "fan-tailed pigeon" or the crop of the "pouter"), is certainly, so far as it goes, on the side of the {117} existence of definite limits to variability.

It is asserted in reply, that physiological conditions of health and life may bar any such further development.

Thus, Mr.Wallace says[109] of these developments: "Variation seems to have reached its limits in these birds.