464 THE GERM -PLASM 



selection a choice of innumerable combinations of the most 

 diversified variations, resulting from the constant minute fluctua- 

 tions of all the units in the germ-plasm. 



Strictly speaking, the process of aiiiphiinixis alone cannot 

 bring about an increase or decrease in the devehpnieftt of a 

 character ; though it may, indeed, establish it more firmly in 

 the germ-plasm by causing an increase in the number of ids, 

 the determinants of which produce the character. An increased 

 development, in the ordinary sense of the term, may, it is true, 

 take place by the extension of a variation over larger areas of 

 the body ; the multiplicity of the ids, and the possibility of the 

 constant production of new idic combinations by the process 

 of amphimixis, accounts for the statement of breeders that the 

 constancy of a character, as well as its increased development, 

 may be affected by selection. The so-called ' individual potency ' 

 must, moreover, be due to the presence of a large number of 

 homodynamous determinants for all the more important char- 

 acters, and it probably results, not only from breeding a race 

 true for a long time, — although this is of course necessary, — but 

 also from favourable combinations of ids being produced by the 

 processes of reducing division and amphimixis. 



Variations do not, however, depend merely on modifications 

 in the composition of a determinant or group of determinants, 

 but frequently result from a doubling or further multiplication 

 of the latter ; and this must also depend primarily on modified 

 external influences, such as those produced by changes in the 

 nutrition of a part of the germ-plasm. The apparently sudden 

 appearance of parts — such as feathers and other epidermic 

 structures, as well as of certain pathological structures, such 

 as the supernumerary fingers and toes of human beings — may 

 be explained in this manner. All such variations do not, indeed, 

 actually arise suddenly, but take place gradually in some of 

 the ids, and only suddenly become apparent when they have 

 accumulated to form a majority. 



The suddenness with which variations appear is, in all proba- 

 bility, only apparent in most cases, as is well shown by 

 Hofi'mann's experiments on wild plants, in which variations 

 were produced by abnormal conditions of life. The degenera- 

 tion of parts which are no longer required, or have simply 

 become useless, is due to the reduction and final disappearance 

 of the corresponding determinants from the germ-plasm. But 



