( cviii ) 



A comparison between the difliculty of producing such a 

 cross and tliat of obtaining hybrids between the Ring Dove 

 and the Rock Pigeon, the ancestor of tlie domestic breeds, 

 would probably throw much light on the Pallasian hypothesis. 



If the view 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 only real 

 evidence when the structural test breaks down, the conclusion 

 is suggested that the real interspecific barrier is not sterility 

 but asyngamy. Nevertheless, as argued on pages civ-cvi, 

 asyngamy 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 selec- 

 tion " of Romanes, in which sterility is supposed to arise 

 spontaneously, asyngamy being not the cause, but the 

 consequence. 



Asyngamy may be brought about in various ways, of which 

 the most obvious is geographical separation. But asyngamy 

 is by no means the necessary result of geographical discon- 

 tinuity or asympatiy. Thus Darwin considered that there 

 is regular inter-breeding between Madeiran and continental 

 birds of the same species. He wrote to Hooker, August 8 

 [I860]: "I do not think it a mystery that birds have not 

 been modified in ^Madeira. Pray look at p. 422 of Origin 

 [ed. iii]. You would not think it a mystery if you had seen 

 the long lists which I have (somewhere) of the birds annually 

 blown, even in flocks, to Madeira. The crossed stock w'ould 

 be the more vigorous." * An even more striking case is that 

 of Pyrameis cardui, which ranges over nearly the whole w'oi'ld. 

 The singular absence of local geographical i-aces in this 

 abundant butterfly is almost certainly due to the astonishing 

 powers of dispersal which enable intermittent syngamy to 

 prevail over the whole vast area of its distribution. 



An interesting and curious cause of persistent asyngamy 

 is the "Mechanical Selection" so thoroughly explained and 

 abundantly illustrated by Karl Jordan. t The complex genital 

 armature of Lepidoptera is during syngamy kept constant by 



* " More Letters," vol. i, pp. -487, 488, Letter 370. 

 t 1. c. p. 518-522. 



