 eey 
A comparison between the difficulty 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 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 36-8, 
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 asympatry. Thus Darwin considered that there 
is regular inter-breeding between Madeiran and continental 
birds of the same species. He wrote to Hooker, August 8 
[1860]. “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 would 
be the more vigorous.” * An even more striking case is that 
of Pyrameis cardui, which ranges over nearly the whole world. 
The singular absence of local geographical races 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.7 The complex genital 
armature of Lepidoptera is during syngamy kept constant by 
* “¢ More Letters,” vol. i, pp. 487, 488, Letter 370. 
+ lc. p. 518-522. 
