164 



SEXUAL DIMORPHISM 



way as to form a longitudinal slit. Why should not the 

 trachea be dilated generally, or form a bulbous enlargement ? 

 For we suppose the pressure of air to have been originally 

 applied in the mature bird with well -developed tracheal 

 rings. 



The answer to this objection could probably be obtained 

 by sufficient investigation. The tracheal rings must have 

 been weaker or less supported by surrounding parts in the 

 place where the opening exists. When a rubber bicycle 

 tyre is inflated strongly it frequently happens that a dilata- 

 tion is formed at a particular limited spot, while the rest of 

 the tube preserves its form. 



On the other hand, it is impossible to conceive of the 

 evolution of the Emeu's pouch by the selection of individual 

 variations apart from the process of inflation. 



Note. In reference to the Oscines or Singing Birds, among 

 the Passeres, it should be mentioned that Dr. A. G. Butler 

 has discovered and described minute differences in wing and 

 bill between male and female in several species. These 

 differences he believes to be related to differences of function 

 inflight and nest -building: they therefore can probably be 

 explained by the principles maintained by me. 



