HISTORY OF ZOOLOGY 



19 



beyond the possibility of a doubt whether in a definite case we ha\e to 

 do with a variety of a species or witli a different species? or do the con- 

 ceptions pass into one another in nature? Are species varieties which 

 have become constant, and are varieties species in the process of 

 formation? 



Morphological Characters. — A. Distinction between Species and^ Variety. 

 For the settlement of these fundamental questions morphological and 



Fig. ia. — English carrier-pigeon (after Darwin). 



Fig. 



-English tumbler-pigeon (after Darwin). 



physiological characters can be considered. In the practice of the system- 

 atists usually the morphological characters prevail exclusively; and 

 hence will be here considered first. If, among a great number of forms 



