RErRESENTATIVE SPECIES. 91 



the rei)resentative race will advance more or less 

 quickly to the higher dignity of a representative 

 species. The difference between form and species 

 is only one of degree. 



There is one more fact which bears very closely 

 on the present subject to which some short allusion 

 must be made, and that is the interesting pheno- 

 menon of discontinuous areas of distribution. So 

 far as is known, a broken area of dispersal is ex- 

 cessively rare; for, as we have already seen, dis- 

 continuity by the isolation of individuals almost 

 invariably leads to a divergence of character. It 

 is therefore most curious to find the members of 

 a species separated into two or more distinct 

 colonies by vast distances continue to preserve their 

 homogeneity absolutely unchanged. One of the 

 best instances known to the present writer is that 

 of the Fork-tailed Petrel {Procellaria leachi). This 

 species breeds on various parts of the coasts of the 

 North Atlantic, from Nova Scotia in the west to the 

 British Islands in the east. It also breeds along 

 the shores of the North Pacific from California to 

 the Aleutian and Kurile Islands and Japan. It will 

 thus be seen that the entire continent of Asia on 

 the one hand and of America on the other separates 

 the two colonies bv a vast land mass which we 



