240 GEOEGE JOHN EOMANES 1886- 



January 21, 1889. 



My dear Sir, I should like you to set your lucid 

 wits to work upon the following questions, and let me 

 know whether you can devise any answers. 



On pp. 220 226 of your book, you state with ex- 

 treme felicity, and much better than he does, Weis- 

 mann's theory of the causes of variation. But it does 

 not occur to him, and does not seem to have occurred 

 to you, that there is a curious and unaccountable 

 interruption in the ascending grades of sexual diffe- 

 rentiation, for in the vegetable kingdom these do not 

 follow the grades of taxonomic ascent ; but, on the 

 contrary, and as a general rule, the lower the order of 

 evolution, the greater is the tendency to bi-sexualism. 

 Dioecious species (i.e. male and female organs on dif- 

 ferent plants) occur in largest proportion among the 

 lower Cryptogams, less frequently among the higher, 

 and more rarely still among Phanerogams. Monoe- 

 cious species (i.e. male and female organs on the same 

 plant, but locally distinct) occur chiefly among the 

 higher Cryptogams and lower Phanerogams ; Herma- 

 phrodite species (i.e. male and female organs in the 

 same flower) occur much more frequently among 

 higher Phanerogams. 



There is, besides, another difficulty. According 

 to Weismann and yourself, it is natural selection that 

 has brought about sexuality ' for the sake of better 

 results in the offspring,' by making them more 

 variable or plastic. But how can natural selection 

 act prophetically ? Unless the variability is of use to 

 the individuals at each stage of its advance, it cannot 



