528 ('olonel i\. Meinertzliagen on [Ibis, 



XXVII. — Some Thovglds on SiiJ'Species and Evolution. 

 By Colonel R. Meinektzhagen, D.S.O., M.B.O.U. 



In 'The Ibis' o£ this year, p. 345, appeared a letter over 

 the signatures of Dr. Lowe and Mr. Mackworth-Praed, in 

 answer to a letter of Mr. Loomis in 'The Ibis' of: 1920, 

 p. 965. 



These letters bring out several points of great importance 

 to those of us interested in evolution, distribution, and 

 migration. These points can be enumerated as follows : — 



1. Is intergradation necessar^^ among forms in order to 



determine their status as species or as geographical 

 races ? 



2. Is a geographical race due to one cause only — environ- 



ment, — or can it be due to mutation ? If the latter is 

 the case, should they not be species ? 



3. Are geographical races incipient species, and can 



geographical races, whose dift'erences are obviously 

 due to environment, ever develop into a species ? 



4. Of what value is the subspecies ? 



5. Definition of a subspecies. 



1. Intergradation. 



Many eminent ornithologists have accepted the fact that 

 absence of intergradation among birds results in a definite 

 entity, the existing species, which must remain the only 

 " definite minor unit in nature." This is })robably correct 

 and covers the vast majority of geographical races, but not 

 all. Insular races, which are obviously but variations due 

 to isolation, having no intermediate terrain with their 

 parent stock, can naturally have no intermediate or inter- 

 graduating ract'S. The same argument applies to variations 

 occurring among isolated continental communities, when the 

 intervening country is unsuited to the life of the species. 

 In these cases isolation is as complete as insular isolation, 

 and the intervening range of hills, desert, or other obstacle 



