ISO MENDEL'S LAWS 



of sex is to mix non-sexual parental characters as colours are 

 blended, or to mix them as marbles are mixed. What, then, is the 

 rule in nature? Is blended ' inheritance' the normal for non- 

 sexual characters and alternative ' inheritance ' abnormal ? Or is 

 alternative ' inheritance ' the normal ? Or are they both normal in 

 the sense that nature has evolved both as adaptations in the 

 sense that both have utility ? This question is being very sharply 

 debated at the present time. Formerly it was universally believed 

 by Darwin for example that offspring tend, as a rule, to blend 

 parental characters, and this view is now upheld by most biologists, 

 and in particular by the biometric or statistical school. The 

 experimental or ' Mendelian ' school, on the other hand, insist 

 that the non-sexual characters of parents are, as a rule, not blended 

 in offspring. 1 



248. Naturally these divergent views of heredity have led to, 

 or are associated with, equally divergent views of evolution. 

 Speaking generally, biometricians maintain that variation and 

 evolution are both continuous ; in other words, they suppose that 

 nature causes evolution by gradually and continuously raising the 

 specific mean through the selection of ' normal ' or ' fluctuating ' 

 variations. 



249. On the other hand, experimental workers, who are usually 

 c Mendelians ' and ' Mutationists,' insist that there are two distinct 

 kinds of variations, the continuous and the discontinuous. The 

 former, they maintain, fluctuate about a mean to which they tend 

 to return so strongly in succeeding generations, that they cannot 

 be made the foundation of permanent racial divergence by any 

 stringency of selection. In other words, they suppose that, owing 



1 It is interesting to note how the different methods by which the rival schools 

 pursue their inquiries have tended to lead to divergent theories of heredity. 

 We shall see presently that blended ' inheritance ' is the rule when parents differ 

 comparatively little, that is, when their differences are of degree rather than 

 of kind ; and that alternative ' inheritance ' is the rule when parental differences 

 are greater, that is when they are of kind rather than of degree. The statis- 

 tician dealing with aggregates of individuals, and noting that even when parental 

 differences are great, offspring tend to ' regress ' towards the parental and specific 

 mean, is impressed by the fact that extreme plus and minus deviations from the 

 average type of the race are rare and are connected by lesser variations. He is 

 inclined, therefore, to lay stress on the truth that the vast majority of variations 

 are ' continuous,' and to believe that offspring tend, as a rule, to blend parental 

 characters. The experimental worker, on the other hand, deals with individuals 

 more than with aggregates, and naturally chooses as materials for study sharply 

 contrasted parental qualities, the inheritance of which can be more easily observed 

 in offspring than mere shades of difference. He tends, therefore, to lay stress on 

 ' discontinuous ' variations, and to believe that alternative inheritance is the rule. 



