l6o CAUSES OF VARIATION 



the centrosome divides before the resting stage, more commonly 

 afterward. Taking it all in all, here is an exceedingly compli- 

 cated procedure, only semi-mechanical and therefore subject to 

 deviations. Absolute constancy demands no failure in the final 

 object of exact qualitative division, but the student sees many 

 possibilities for unequal division and therefore for deviation in 

 growth. Is this the fundamental cause of mutations ? One thing 

 is certain, living forms are made up of elements, and these 

 elements are subject to strange combinations throughout the 

 entire range of plant and animal life, and the facts seem to teach 

 that from time to time combinations may arise that are entirely 

 new. Moreover, whole units seem occasionally to be " lost out," 

 as when horned cattle suddenly give rise to polled strains, hairy 

 species to smooth varieties, colored to albino, etc. 



Conversely, do vital elements like chemical radicles assume 

 new combinations from time to time, giving rise to new char- 

 acters and new types which we call " mutants " ? We do not 

 know, and yet we feel the conviction that at this point we are 

 very close to the " origin of characters," the cause of mutations 

 and of variation in general. 



Manifestly, in so far as irregularities in cell division may be a 

 cause of variation, the matter lies absolutely beyond our control 

 except that lines in which it is believed to occur may be avoided 

 in selection. Here is a field, however, too far beyond our pres- 

 ent knowledge to admit of anything more than the merest 

 mention. We confidently believe that the future will shed more 

 light on this obscure subject. 



SECTION II BISEXUAL REPRODUCTION A FUNDAMENTAL 

 CAUSE OF VARIATION 



Among higher animals and plants the new individual is the 

 direct product of two others, the male and the female parent, 

 -and is of necessity different from either, being a product of 

 both. In bisexual reproduction, therefore, biologists recognize a 

 fundamental cause of variation, slight if the parents are of like 

 blood lines, extreme if of radically different, as in hybridism. 



