INTERNAL CAUSES OF VARIATION 163 



of introducing new characters into the race, and while it is mani- 

 festly a fruitful source of never-ending combinations of racial 

 characters in new individuals, yet variations so introduced are 

 comparatively slight except when the two parents belong to sepa- 

 rate lines. 



Fertilization of the ovum is something more than a stimulus 

 to growth. It is a real union of material bodies, physiological 

 units, or whatever they may be called, representing the hereditary 

 substance of both parents. Bisexual reproduction is therefore 

 not only a guaranty of transmission of racial characters but also 

 an assurance of inheritance with some variation. 



Control. Here is a fundamental cause of variation practically 

 under the control of the breeder through selection. True, his 

 knowledge and his judgment are insufficient to insure him against 

 mistakes in mating, and it is also true that there are many other 

 influences at work to produce variations, but this is the field in 

 which the breeder can exert the largest influence, and it is by 

 selection that the greatest results in improvement have been 

 attained up to date. 



Sexual selection, 1 preferential mating, 2 and assortative mating. 3 

 Powerful as are these influences in directing the trend of varia- 

 bility, they yet belong to general evolution because they are ele- 

 ments in natural selection, and they have no place in the present 

 discussion. 



SECTION III MATURATION AND THE REDUCTION OF 

 THE CHROMOSOMES A CAUSE OF VARIATION 



Fertilization is a process whose inevitable consequence would 

 seem to be the "piling up " of nuclear matter indefinitely ; for 

 if, with each new generation, the chromosomes (or physiological 

 units) of the one parent are added to those of the other, it would 

 seem that in time the resulting nuclear matter would speedily 

 become " unmanageably large " and inconceivably complex, an 

 event certain to follow except for a series of very remarkable 



1 Darwin, Origin of Species, see Index. 



2 Pearson, Grammar of Science, pp. 425-428. 



3 Ibid. pp. 429-437. 



