Fluctuations and Mutations 73 



duced at will : no sowings of thousands or even of millions of plants 

 will induce them, although by such means the chance of their 

 occurring will obviously be increased. But they are known to occur, 

 and to occur suddenly and abruptly. They have been observed 

 especially in horticulture, where they are ranged in the large and 

 ill-defined group called sports. Korschinsky has collected all the 

 evidence which horticultural literature affords on this point 1 . Several 

 cases of the first appearance of a horticultural novelty have been 

 recorded: this has always happened in the same way; it appealed 

 suddenly and unexpectedly without any definite relation to previously 

 existing variability. Dwarf types are one of the commonest and 

 most favourite varieties of flowering plants ; they are not originated 

 by a repeated selection of the smallest specimens, but appear at 

 once, without intermediates and without any previous indication. 

 In many instances they are only about half the height of the original 

 type, thus constituting obvious novelties. So it is in other cases 

 described by Korschinsky : these sports or mutations are now recog- 

 nised to be the main source of varieties of horticultural plants. 



As already stated, I do not pretend that the production of horti- 

 cultural novelties is the prototype of the origin of new species in 

 nature. I assume that they are, as a rule, derived from the parent 

 species by the loss of some organ or quality, whereas the main lines 

 of the evolution of the animal and vegetable kingdom are of course 

 determined by progressive changes. Darwin himself has often pointed 

 out this difference. But the saltatory origin of horticultural novelties 

 is as yet the simplest parallel for natural mutations, since it relates to 

 forms and phenomena, best known to the general student of evolution. 



The point which I wish to insist upon is this. The difference 

 between small and ever present fluctuations and rare and more 

 sudden variations was clear to Darwin, although the facts known 

 at his time were too meagre to enable a sharp line to be drawn 

 between these two great classes of variability. Since Darwin's time 

 evidence, which proves the correctness of his view, has accumulated 

 with increasing rapidity. Fluctuations constitute one type; they 

 are never absent and follow the law of chance, but they do not afford 

 the material from which to build new species. Mutations, on the 

 other hand, only happen to occur from time to time. They do not 

 necessarily produce greater changes than fluctuations, but such as may 

 become, or rather are from their very nature, constant. It is this con- 

 stancy which is the mark of specific characters, and on this basis every 

 new specific character may be assumed to have arisen by mutation. 



Some authors have tried to show that the theory of mutation is 

 opposed to Darwin's views. But this is erroneous. On the contrary. 



1 S. Korschinsky, " Heterogenesis und Evolution," Flor,i, Vol. lxxxix. pp. 240 — 363, 1901. 



