GENETICS 169 



species, with one-sided sex-determination (female pre- 

 ponderance, parthenogenesis, etc.), and frequently with 

 extinction. Mendelian segregation may frequently be no more 

 than the expression of incompatibility, of mutual (physio- 

 logical) infertility, under certain circumstances. It may thus 

 be a means of progress, in the same way as the poisonous effect 

 of pollen on the stigma sometimes amounts to mutual 

 infertility and prevents in and in-breeding, and stimulates 

 instead cross-fertilisation, which is beneficial. Whether 

 segregation is thus to subserve mutual infertility as a part of 

 a continuous process, i.e., the formation of distinct physio- 

 logical species or not, depends on the conditions of life. 



Mendelian segregation thus seems to indicate the 

 significance of the fertilisation process. 



Prof. Bateson, who as the leader of the Cambridge School 

 of Genetics has done so much to develop Mendelian ideas, 

 states that it seems surprising that a feature apparently so 

 fundamental as sex, i.e., reproductive activity, should be 

 differently constituted in different animals. 



The simple explanation is that the differences of sex life 

 are constituted in accordance with a factor that may easily be 

 seen to make for differentiation generally, i.e., bio-economic 

 specialisation, as we have seen in the case of sexual symbiosis 

 in plantagens, particularly in that of the lichen, and as we 

 shall presently see in the case of cross- versus self-fertilisation. 



"In the case of Variations generally," says Dr. Walker, 

 " which are of sufficient advantage to the race to be preserved 

 by Selection, the alternative inheritance disappears in time 

 and the character becomes racial." 



This, of course, may mean anything. "Selection" is a 

 very convenient word. We have seen that in pathological cases 

 some of which have a wide distribution a very "bad" 

 variation becomes fixed and drives out a "good" variation, 

 much in the same way as bad currency in human politics drives 

 out good currency. This is indeed the great danger of all 

 organic life, that it allows of pathological as well as healthy 



