OTHER THEORIES OF SPECIES-FORMING. 35 L 



"4. The new species that appear may be in some cases 

 already adapted to live in a different environment from that 

 occupied by the parent form; and if so, it will be isolated 

 from the beginning, which will be an advantage in avoiding 

 the bad effects of intercrossing. 



"5. It is well known that the differences between related 

 species consist largely in differences of unimportant organs, 

 and this is in harmony with the mutation theory, but one 

 of the real difficulties of the selection theory. 



"6. Useless or even slightly injurious characters may 

 appear as mutations, and if they do not seriously affect the 

 perpetuation of the race, they may persist." 



Finally, the attention of students especially may be called 



to Bateson's interesting suggestion that mutations may be 



Bateson's Bug- simply pure Mendelian recessives appearing 



gestion that mu- after a crossing. It would take us too far 



tations are - , , A . . . , 



Mendelian re- afield to attempt to explain here to readers 

 cessives. unacquainted with the Mendelian principles 



of inheritance just how Bateson's suggestion has a certain 

 plausibility. It must suffice to say that Mendel, and after 

 him a considerable number of present-day students of 

 heredity, have shown that after a crossing between two- 

 individuals sharply contrasting in regard to some particular 

 character, as colour of hair, all the offspring of the first 

 generation may agree in showing but one of the two parental 

 colours (the dominant), but that if these first generation 

 'offspring are bred to each other, or to similarly produced in- 

 dividuals, the members of the second generation will split 

 up as regards the character in question, some showing one 

 of the grand-parental hair colours, and the rest showing the 

 other one. Now breeding likes together, it would be shown 

 in third generation groups that one of these colours, and, 

 namely, that one called the recessive, which did not appear 

 at all in the first generation, will always henceforth breed 

 true while the other colour may or may not breed true (de- 



