Page:Popular Science Monthly Volume 62.djvu/493

Rh, do not exist for Scott. Each species remains unchanged as long as its period of existence lasts. All its characters vary more or less according to the law of Quetelet, but the type, to which all variations return, remains the same through centuries.

A species changes only when it produces others. Or rather, it does not change, but continues to exist next to the species newly formed. It may be compared to a tree, which, though it produces branches, does not cease growing in length. Only when among its descendants there are types better fitted for the battle of life, a species may locally succumb. But it would require a long time before the new species had entirely taken the place of the old.

It is clear that one must distinguish by some simple term variation by jumps from variation obeying the law of Quetelet. It is not practical to use the terms, sport, discontinuous variation or spontaneous variation, since they tend to produce the impression of something incomprehensible. Scott did not use these terms. He speaks of 'mutations.' A mutation occurs when one species is formed from another. As it is, 'mutation' is the expression in general use before the days of Darwin, and at first used by Darwin himself. Since it has apparently fallen into disuse, except in paleontology, where it is met with in various authors, always conveying the same meaning, it seems best to continue to use this term. Hence as long as species produce others they are called mutable, and this part of the doctrine of variability is known as mutability.

Once it has been conceded that species originate from others by mutation, one can go on to investigate what deductions must be made in regard to this process from the facts with which we are acquainted. And as long as an empirical investigation was impossible it was of the utmost importance to be able, even in this manner, to form an opinion about it.

First of all, we can come to the conclusion that mutations must be the smallest changes which can produce a difference between two species or rather between two constant types. Ordinarily the estimation of the differences existing between two related species is too great. Differences as between a horse and a donkey are of course not the result of a single mutation; there must have been a series of transition forms, at present extinct. Nobody will expect to see so great a change occur at once. Even much slighter differences, for instance those existing between our native violets, are still too large; here also there must have existed transition forms. And indeed a comparison with the floras of other countries actually does show a number of forms which bridge over these differences.