294 Distribution of Moths of the Sub-family Bistonince. 



headquarters in Eastern Asia ; secondly, the genus Biston 

 reaches its highest pitch of development in Asia ; thirdly, 

 there are no true Bistons in the intervening tracts in Asia as 

 there would have been infallibly had Biston been of European, 

 and therefore of recent, evolution ; lastly, had Biston originated 

 in Europe it is exceedingly difficult to see how strataria could 

 have reached Algiers and Morocco in the limited time at its 

 disposal with the causeways in their present broken-down 

 condition. 



In truth, all of these arguments point unwaveringly to one 

 conclusion, and that is that Biston, like all its immediate allies, 

 is a genus of far Eastern antecedents. Many facts of small 

 import singly (some of which have been stated), but of com- 

 pelling force when in bulk lead us to that decision. 



Having thus, in our own minds, fixed its home in the far 

 East, we must now determine when it appeared. Its failure 

 in America gives us a starting point. Had it been a dweller 

 in the old Miocene continent then, of a surety, we should 

 have found it there. Again, Phigalia has been derived from 

 some link between Amphidasys and Biston, as many characters, 

 pupal, antennal and otherwise suggest, and Phigalia, as we know 

 managed to reach America by the Northern route ; whence the 

 necessary conclusion is that Biston came into being at some time 

 between the close of the Miocene Period and times when the 

 Northern path was under such climatic conditions as allowed 

 Phigalia to pass but prevented Biston, a genus more fastidious 

 in its climatic requirements. These times, with due respect 

 to its inability to pass to America, were not long prior to the 

 middle of the Pliocene Epoch. 



Additional evidence of quite a different type gives ample 

 confirmation of this judgment. Japan, as we see above, 

 produces one endemic species so that the genus cannot have 

 been produced since Japan became an island. Moreover, it 

 has crept into India, which suggests that it passed when the 

 Himalayas were less of a barrier than at present and, lastly, 

 we find it in Africa. From this evidence, built up link by link, 

 but one conclusion is tenable, and that the same as arrived at 

 before, that Biston put in an appearance in the first half of the 

 Pliocene Period. 



Couple the facts of its occurrence in North Africa and 

 Spain with its Eastern origin, and we perceive that, in our 

 own B. strataria, we have a representative of the old familiar 

 Oriental, as opposed to the later Siberian migration, which, 

 in those early times, whether by direct or by devious paths, 

 was a weighty factor in the populating of Europe. 



Unless we encounter here a case of converging evolution, 

 the nearness of B. robustum in the female sex to the genera 

 Megabiston and Lycia would indicate that B. robustum has 



Naturalist, 



