Page:Popular Science Monthly Volume 64.djvu/459

Rh instances of distinctly prepotent variations are known, and such were taken by Mivart and other zoologists to prove that species do not originate by gradual change, but abruptly or by 'extraordinary births,' a view quite similar to the recently published 'Theory of Mutations,' but distinctly more practical because the 'mutations' of plants which are the basis of the inferences of Professor De Vries are not prepotent but 'recessive,' presumably because they do not represent true genetic variations, but are symptoms of what may be described as an evolutionary debility, due to inbreeding. The disappearance of mutative characters when the new variations are crossed with the parent form or with each other is merely the recovery, as it were, of the health of the species when the abnormal condition of inbreeding has been removed, as shown so conclusively in Darwin's well-known experiments with pigeons, and confirmed by an abundance of similar facts.

Though differently interpreted, many other facts supporting this view were collected by Darwin, who summarized the results of his studies of Ipomea, Digitalis, Origanum, Viola, Bartonia, Canna and the common cabbage and pea, as follows:

"The most important conclusion at which I have arrived is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution, owing to their progenitors having been subjected during several generations to slightly different conditions, or to what we call in our ignorance spontaneous variations."

Differences between the plants of different habitats mean also different lines of descent and attendant variations, and the beneficial results of bringing these together may be explained by reference to symbasis rather than to the 'slightly different conditions.'

While it may not be insisted that species, as described and named by systematists, are never originated by 'extraordinary births,' or from 'mutations,' both suppositions are obviously improbable as general explanations. Mutations are seldom fitted to survive because they are less vigorous and less fertile than the parent type, so that they must be segregated at once in order to be preserved. And even prepotent variations have no necessary connection with the origination of species, since however rapidly the characters of a species might change, it would still be the same species until a subdivision had taken place. The more a species evolves the more different from its relatives it becomes, and the more satisfactory for the purposes of systematic study, but this progressive transformation of the 'type' carries with it no necessity for subdivision, nor any indication that evolution is concerned with the origination of species.