84 WHAT IS A SPECIES:^' 



thib male was mated with a Blue Homer hen, which 

 produced healthy offspring.^ 



A comparison between the difficult)- of producing such 

 a cross and that of obtaining hybrids between the Ring 

 Dove and the Rock Pigeon, the ancestor of the domestic 

 breeds, would prol:)abl}- throw much light on the Pallasian 

 hypothesis. 



If the \iew here proposed be sound that Syngamy 

 lies behind, and is at least provisionally implied in the 

 transition which means so much to the systematist, and is 

 his onl\- real evidence when the structural test breaks 

 down, the conclusion is suggested that the real inter- 

 sj)ecific barrier is not sterility but As}ngamy. Neverthe- 

 less, as argued on pj). 80. 81, As)ngamy will infallibly 

 lead to sterility, although the result may be long delayed. 

 This latter view, which was that of Darwin, is the exact 

 opposite of the ' physiological selection ' of Romanes, 

 in which sterility is supposed to arise spontaneousK', 

 Asyngam)' being not the cause, but the conse(juence. 



Asyiij^'aniy as a LonsL'(puiuc of Asympatry. 



Asyngamy ma}' be brought about in various ways, 

 of which the most obvious is geographical separation. 

 But Asyngani)- is by no means the necessary result of 

 geographical discontinuity' or Asymj)atr) . Thus Darwin 

 considered that there is regular interbreeding between 

 Madciran and continental birds of the same species. He 

 wrote to Hooker, August 8. |i866|. 'I do not think 

 it a mystery that birds have not been modified in Madeira. 

 Pray look at p. 422 of Oric^in [ed. iii]. \o\\ would not 

 think it a m\stery if )ou had seen the long lists which 

 I have (somewhere) of the birds annually blown, even in 

 flocks, to Madeira. The crossed stock would be the 

 more vigorous.' - An even more striking case is that 



' The Zoologist, November, 1903, p. 401. 



' Morf Letters, vol. i, pp. 487. 488, LeUer 370. Dr. Karl Jordan has 

 maintained (' Der Gegensatz zwischen geograpliibcher und nichtgeo- 

 graphischer Variation '. Zeitschr. f. Wisscnschaft. Zool. Bd. Ixxxiii) that 

 this argument is valueless, inasmuch as more critical study of extensive 



