436 THE GERM- PLASM 



which a given structure in a species has suddenly become 

 considerably modified. How far* such modifications have to 

 do with the formation of new species need not concern us at 

 present : w^e have only to consider the causes and method of 

 origin of these modifications. 



A large number of cases of this kind have been observed, 

 especially amongst plants. Not only fruZ/s, but leaves, blossoms, 

 and entire shoots have been found to vary suddenly, and in 

 a striking manner. Several varieties of fruits must be included 

 in this category — such as the variety of peach known as the 

 nectarine, as well as the moss-rose, copper-beech, copper-hazel, 

 the varieties of the beech, hornbeam, and oak, the fern-leaved 

 variety of the maple, and numerous other plants cultivated in 

 our gardens. 



These varieties in some cases first appeared as seedlings, i-e., 

 as entire plants, and in others as simple branches or shoots, in 

 which latter case they are usually known as bud variations . 



Let us first take into consideration the varieties which 

 have originated from seeds. These occur most frequently in 

 cultivated plants, i.e., in species which have existed for some 

 length of time under conditions which differ more or less from 

 the natural ones. We are therefore undoubtedly justified in 

 attributing the cause of the variation to the influence of changed 

 external surroundings. But a wild plant, which is transplanted 

 into a garden soil, does not always begin to vary at once : it 

 has, in fact, been shown by Hoffmann's experiments, of which 

 we have already given an account, that many generations often 

 elapse before conspicuous variations occur. And even then they 

 do not appear in all the seedlings, occurring perhaps only in one 

 among several hundreds or thousands. 



As in the case of ordinary individual variations of a minor kind, 

 the modification begins a long time before it becomes apparent. 

 A few determinants are first changed, and then a gradually 

 increasing numl^er, until at last, by means of the ' reducing divi- 

 sion' and amphimixis, they occur in such numbers in certain 

 germ-cells that they form the majority. The fact that these 

 variations occur on a larger scale than the ordinary ones is due 

 to the permanent action of uniform changes in nutrition, which 

 give a constant direction to the modification of susceptible 

 determinants, so that an increase is eflfected. This is wanting 

 in the case of the ordinary and constantly varying nutrition. 



