334 SCIENCE PROGRESS 



minants of sex, described by both Profs. Davenport and 

 Thomson, I think that the conclusion may be arrived at on 

 more general lines. Such differences as constitute sex, funda- 

 mentally the difference between the production of cells that 

 actively fertilise and those that are passively fertilised, must, 

 like other characters, have arisen from variations that were 

 transmitted in an alternative manner. In the case of variations 

 generally which are of sufficient advantage to the race to be pre- 

 served by selection, the alternative inheritance disappears in 

 time and the character becomes racial. But the advantages of 

 the differentiation of individuals into two sexes is dependent 

 upon the alternative occurrence of particular characters, so 

 selection would necessarily have eliminated the tendency to 

 blend to a great extent. That it has not done so beyond the 

 necessary point is evident from the potentiality of producing 

 the secondary characters of the opposite sex under certain con- 

 ditions, a potentiality which varies in different individuals just 

 as do all other characters. Thus we see that, as Prof. Daven- 

 port says, opposite characters when crossed always leave their 

 marks upon each other when extracted ; and also we see that 

 the variation towards blending is always appearing, which fact 

 Prof. Davenport has missed. 



I must confess that I am unable to follow the argument of 

 Prof. Thomson, who says that " the difference between an ovum 

 producer and a sperm producer is fundamentally a difference in 

 the balance of chemical changes, i.e. in the ratio of anabolic and 

 katabolic processes." Why should not the difference in the 

 " ratio of anabolic and katabolic processes " be the result, not 

 the cause, of sexual differences ? 



A comparatively recent and serious cause of contention has 

 arisen out of de Vries' mutation hypothesis. In de Vries' own 

 words, quoted by Prof. Thomson, this may be briefly described 

 as follows : " The current belief assumes that species are slowly 

 changed into new types. In contradiction to this conception 

 the theory of mutation assumes that the new species and 

 varieties are produced from existing forms by sudden leaps. 

 The parent type itself remains unchanged throughout this pro- 

 cess and may repeatedly give rise to new forms." Prof. Thom- 

 son has such a high opinion of this hypothesis that he constantly 

 treats it as though it were generally accepted by biologists all 

 over the world. It certainly accords well with the tendency he 



