54 ANIMAL LIFE AND HUMAN PROGRESS 



their origin to a germinal modification. But how can we 

 explain their abrupt arrival on the scene ? 



It has been proved that, in a great many cases, the exhibi- 

 tion of a " mutant " is due to the combination of factors pre- 

 viously segregated in different individuals of a population. 

 We have seen that there are " modifying " factors, which only 

 produce an effect when brought into combination with two, 

 three, or even more factors which, in the absence of the 

 modifier, give quite a different effect. Experiments, supported 

 by mathematical calculation, have shown that a single factor 

 may be so isolated in the course of breeding that it survives 

 only in a very small percentage of the individuals composing 

 a general population. And many mutants owe their origin to 

 the mating of an individual possessing such a modifier in its 

 germ-plasm with another individual having the appropriate 

 factors on which the modifier can work. 



The reproduction of a grey mouse from the cross albino and 

 waltzing mouse illustrates this point, as well as giving an 

 explanation of the puzzling problem of reversion. It further 

 illustrates the fact that factors do get separated in different 

 individuals, for both albino and waltzing mice were undoubtedly 

 originally derived from ordinary grey mice, and the one race 

 has lost factors which the other has retained, and vice versa. 



Professor Morgan is inclined to the opinion that all muta- 

 tions are due to the recombination of such isolated factors, and 

 that the improvement of a race by selection is a process of 

 picking up a series of modifiers by selective breeding. 



But this view presents great difficulties. It involves the 

 belief that every species was, at some time, endowed with a 

 stock of factors, which can be sorted out and recombined again 

 by the mechanism of heredity, but are themselves unchange- 

 able. If this were so, it would be impossible to account for 

 all the variety of the animal and vegetable world. 



The alternative view is that factors are changeable ; and the 

 most probable view is that they are changed in consequence of 

 some physical or chemical alteration in their environment, 

 which is the " soma " or body of the individual in which they 

 are carried. 



