io PLANT-BREEDING 



only from time to time. A parent species may produce its 

 offspring separately at intervals, or in larger numbers during 

 distinct mutating periods. After this production, the old 

 species is still the same as it was before, and it subsists in 

 the midst of its children. New forms are produced by the 

 old, either in one, or a few, or in numerous individuals; in 

 the latter case, the chance of survival is evidently enhanced. 

 Some young species will be better fitted for their life- 

 conditions than others, and the struggle for life will induce 

 a selection among them by which the fittest survive. Even 

 as the new species are produced locally and as the effect 

 of local causes, the struggle for life and natural selection 

 decide concerning the survival according to the local con- 

 ditions. These conditions thus have a twofold significance 

 for the development of the pedigree of the main groups of 

 plants and animals, but it is probable that they determine 

 the lines of progress chiefly by their selective activity. 



The main arguments in the discussion of the production 

 of species by slow changes or by mutations were taken by 

 Darwin from the experience of agricultural and horticul- 

 tural breeders. Therefore it is desirable to inquire into 

 their real significance. Do they support the one or the 

 the other view? Darwin assumed that they gave proof of 

 slow changes, and took his arguments mainly from the agri- 

 cultural side. In horticulture, however, as we have seen 

 in discussing Korshinsky's work, the probability is on the 

 other side. In my experiments on mutability I have shown 

 that it is possible to repeat and control the origin of horti- 

 cultural and analogous varieties under strict experimental 

 precautions, and that the full proof may be given that they 

 originate at once, and not by a slow process of changes. 

 They may, in the first instance, appear with the full display 

 of their average character, or only with a small indication 

 of it as an extreme variant of its fluctuation, but in the latter 



