Page:The Effect of Research in Genetics on the Art of Breeding (1912).djvu/5

Rh a unit character may be apparently lost in crossing, owing to the prevailing presence of its dominant allelomorph, but that in reality it is not lost or apparently changed and will reappear again when it happens that two gametes both bearing the character meet in fecundation. It may remain hidden for many years, but as we are now inclined to view the matter, the character or the determiner of the character would not be permanently lost to the species unless all individuals possessing it were killed before they produced seed. This unit character idea would lead us to the conception of the species as made up of all the unit characters that it has acquired by any means in its development and which still exist. The acquirement of any new unit character would add one more character to the species and double the number of possible varieties or races of the species.

In evolutionary studies we have long recognized that variation was the foundation of evolution and that no evolution was possible without variation, but we have assigned to selection an all-important part as guiding and even stimulating the variation in a certain direction. Darwin and particularly some of his more radical followers have assigned to selection a creative force, in that it has been assumed that when nature by a slight variation gave the hint of a possible change in a certain direction, natural or artificial selection, by choosing this variant and selecting from among its progeny the most markedly similar variants, could force the advance of the variation in the direction indicated. Since Darwin’s time this cumulative action of selection has been emphasized so forcibly that we had come to recognize selection as an active force in creation rather than simply as a selective agency. To be the vital principle of evolution, as we now understand the species as made up of heritable unit characters, the selectionist must show that a new character can be created by selection, otherwise selection becomes a secondary principle.

When viewed from the standpoint of the production of a new and definitely heritable unit which mendelizes, the task of selection becomes more doubtful. Darwin’s idea, that changes in species required many years and probably many centuries for accomplishment, took the subject largely out of the field of experimentation and in a measure developed a speculative science. One of the greatest contributions to science made by De Vries was to establish the study of evolution on an experimental basis. With the demonstration that evolution could be studied experimentally, the question of the effectiveness of selection was taken up, and we are now doubtless on the road to a solution of the problem. It is only possible for us here to call attention to a few of the researches in this direction.

The classical researches of De Vries, now familiar to us all, challenged the correctness of the selection theory and sought to show that species originated by sudden jumps or mutations. We may admit that De Vries proved that species or new characters were formed suddenly as mutations, but this would not prove that they might not also be formed or actually induced to mutate by a continuous process of selection. Indeed, in his experiments on the production of a double-flowered variety of Chrysanthemum segetum (“Mutationstheorie,” Vol. I., p. 523), a few generations of selection led to markedly increasing the number of ray-florets before the ligulate corollas appeared among the disk-florets, the change which he interpreted as the mutation that gave him the double variety.

Johannsen has contributed much to our knowledge of selection and has given us a more exact method of experimentation by