CHAPTER XIII 



INFLUENCE OF SELECTION 



Artificial selection is par excellence an experimental pro- 

 cess, and has been applied with success to all of the known forms 

 of variation. It has been used in the case of elementary varieties, 

 that follow the law of discontinuous variation and inheritance 

 as described in the preceding chapters. Whenever, in these 

 cases, Mendel's law holds, the formation of "pure" races can 

 be quickly brought about by selecting the extracted dominants 

 and extracted recessives. Where more than a single character 

 is involved, the formation of a new race by selecting individuals 

 containing two, three, or more desirable characters is more in- 

 volved, and yet can be carried out with certainty. Selection 

 produces nothing new in such cases, except in forming new com- 

 binations of already existing characters. 



Selection may be applied to elementary species, which are new 

 forms or sports that present one or more new characters. In 

 this case isolation is the conserving principle of most impor- 

 tance; but in cases where a single individual alone comes into 

 existence, it must be crossed with the parent type, and from the 

 offspring obtained selection of those that have the new character 

 must be made. If the new character is transmitted to all the 

 offspring, no selection amongst them may be necessary; but if 

 the new character follows the Mendelian law of sphtting in the 

 second generation, selection and isolation may be necessary to 

 obtain as quickly as possible a new race. In this form of selec- 

 tion, also, nothing new is created, but what has appeared "spon- 

 taneously" is preserved. 



The third form of selection is that applied to fluctuating 

 variations. In the present chapter this topic especially will be 



