EVOLUTION OF THE COLORS OF BIRDS. 115 



for the first time elaborated it and pointed out some of 

 its bearings upon the doctrine of natural selection. Mr. 

 E,. Meldola was one of the first critics of the new theory.* 

 He asserted that the principal difficulty pointed out by 

 Mr. Romanes was " the sterility of natural species as 

 compared with the fertility of domesticated races." In- 

 asmuch as the struggle for existence is most intense 

 between those individuals which are most alike, any- 

 thing which would cause them to vary would be an 

 advantage which natural selection would make use of. 

 The prevention of intercrossing by inter-sterility would 

 be such an advantage and natural selection would 

 accordingly favor it. 



Mr. Francis Galton, soon after offered another sug- 

 gestion regarding physiological selection.! According 

 to his view certain individuals of a race are isolated 

 from the rest by having similar sexual likes and dislikes 

 which cause them to interbreed rather than to mingle 

 with the rest of the tribe. In replying to the above two 

 criticisms, Mr. Eomanes attempts to show that neither 

 Galton nor Meldola have really opposed his views. Mr. 

 Galton had objected to physiological selection on the 

 ground that the intersterility which it presupposes would 

 not be externally apparent, and consequently many in- 

 fertile unions would result. There is nothing to show, 

 however, that infertile unions do not take place at times, 

 and Mr. Galto.n's hypothesis of psychological selection 

 simply supplements among the higher animals the more 

 universal factor of physiological selection. 



In replying to Mr. Meldola's criticism, Mr. Romanes 

 shows that it is not his theory of physiological selection 

 which is called in question, but simply whether natural 

 selection may not in every case underlie and condition 



* Nature xxxiv, pp. 384-395. 

 1 1.0. pp. 395-396. 



