species of plant, nor for the amount of variation it has undergone, nor will it indicate the time when it first appeared, nor the form it had when created.'[1]
To what an extent the limits of species are indefinable, is evinced, he says, by the singular fact that, among those botanists who believe them to be immutable, the number of flowering plants is by some assumed to be 80,000, and by others over 150,000. The general limitation of species to certain areas, suggests the idea that each of them, with all their varieties, have sprung from a common parent, and have spread in various directions from a common centre. The frequency also of the grouping of genera within certain geographical limits, is in favour of the same law, although the migration of species may sometimes cause apparent exceptions to the rule, and make the same types appear to have originated independently at different spots.[2]
Certain genera of plants, which like the brambles, roses, and willows in Europe, consist of a continuous series of varieties, between the terms of which no intermediate forms can be intercalated, may be supposed to be on the increase, and therefore undergoing much variation; whereas genera which present no such perplexing gradations, may be those which have been losing species and varieties by extinction. The annihilation of the intermediate forms which once existed, makes it an easy task to distinguish those which remain.
It had usually been supposed by the advocates of the immutability of species, that domesticated races, if allowed to run wild, always revert to their parent type. Mr. Wallace had said in reply, that a domesticated species, if it loses the protection of man, can only stand its ground in a wild state by resuming those habits, and recovering those attributes
