Mendel’s results have received confirmation by a number of more recent workers, and while in some cases the results appear to be complicated by other factors, yet there can remain little doubt that Mendel has discovered one of the fundamental laws of heredity.
It has been found that there are some cases in which the sort of inheritance postulated by Mendel’s law does not seem to hold, and, in fact, Mendel himself spoke of such cases. He found that some kinds of hybrids do not break up in later generations into the parent forms. He also points out that in cases of discontinuity the variations in each character must be separately regarded. In most experiments in crossing, forms are chosen which differ from each other in a multitude of characters, some of which are continuous and others discontinuous, some capable of blending with their contraries while others are not. The observer in attempting to discover any regularity is confused by the complications thus introduced. Mendel’s law could only appear in such cases by the use of an overwhelming number of examples which are beyond the possibilities of experiment.[[25]]
[25]. This statement is largely taken from Bateson’s book.
Let us now examine the bearing of these discoveries on the questions of variation which were raised in the preceding pages. It should be pointed out, however, that it would be premature to do more than indicate, in the most general way, the application of these conclusions. The chief value of Mendel’s results lies in their relation to the theory of inheritance rather than to that of evolution.
In the first place, Mendel’s results indicate that we cannot make any such sharp distinction as Darwin does between the results of inheritance of discontinuous and of continuous variations. As Mendel’s results show, it is the separate characters that must be considered in each case, and not simply the sum total of characters.
The more general objection that Darwin has made may appear to hold, nevertheless. He thinks that the evolution of animals and plants cannot rest primarily on the appearance of discontinuous variations, because they occur rarely and would be swamped by intercrossing. If Mendel’s law applies to such cases, that is, if a cross were made between such a sport and the original form, the hybrid in this case, if self-fertilized, would begin to split up into the two original forms. But, on the other hand, it could very rarely happen that the hybrid did fertilize its own eggs, and, unless this occurred, the hybrid, by crossing with the parent forms in each generation, would soon lose all its characters inherited from its “sport” ancestor. Unless, therefore, other individuals gave rise to sports at the same time, there would be little chance of producing new species in this way. We see then that discontinuity in itself, unless it involved infertility with the parent species, of which there is no evidence, cannot be made the basis for a theory of evolution, any more than can individual differences, for the swamping effect of intercrossing would in both cases soon obliterate the new form. If, however, a species begins to give rise to a large number of individuals of the same kind through a process of discontinuous variation, then it may happen that a new form may establish itself, either because it is adapted to live under conditions somewhat different from the parent form, so that the dangers of intercrossing are lessened, or because the new form may absorb the old one. It is also clear, from what has gone before, that the new form can only cease to be fertile with the parent form, or with its sister forms, after it has undergone such a number of changes that it is no longer able to combine the differences in a new individual. This result will depend both on the kinds of the new characters, as well as the amounts of their difference. This brings us to a consideration of the results of De Vries, who has studied the first steps in the formation of new species in the “mutations” of the evening primrose.
The Mutation Theory of De Vries
De Vries defines the mutation theory as the conception that “the characters of the organism are made up of elements (‘Einheiten’) that are sharply separated from each other. These elements can be combined in groups, and in related species the same combinations of elements recur. Transitional forms like those that are so common in the external features of animals and plants do not exist between the elements themselves, any more than they do between the elements of the chemist.”
This principle leads, De Vries says, in the domain of the descent theory to the conception that species have arisen from each other, not continuously, but by steps. Each new step results from a new combination as compared with the old one, and the new forms are thereby completely and sharply separated from the species from which they have come. The new species is all at once there; it has arisen from the parent form without visible preparation and without transitional steps.