63 



circumstances of climate would follow on. If we compare our 

 more generally distributed species with those inhabiting Con- 

 tinental Europe, we find the majority of them are identical, 

 and we arrive at the conclusion that they are of common 

 origin, and that we owe our present insect fauna to a gradual 

 extension of range from that direction. This would appear 

 to indicate an initial migratory condition which it is easily 

 conceivable would be subject to such modification as became 

 necessary from time to time in the economy of a species, and 

 the cutting off of the British Islands from the mainland of 

 Continental Europe cannot be regarded as imposing an 

 obstacle that would not be easily overcome by migratory 

 instincts thus engendered. 



If, however, the sphere of our observations is confined 

 entirely to these islands, we have great difficulty in detecting 

 cases of probable migration tliat may not be referable to other 

 causes ; but if we find insects moving in numbers, in situations 

 that preclude the possibility of their presence being the result 

 of any influences of the immediate neighbourhood in which 

 we find them, we can arrive at no other conclusion than that 

 they have migrated from some other position. A very 

 striking case of this description has recently been recorded 

 {E.M.M. xxii. 12). The sailing vessel " Pleione " was on a 

 homeward voyage from Wellington (N.Z.), not having touched 

 at any later port, and on March 27th, 1885, when in lat. 0° 

 47" N., long. 32° 50" W., almost in mid-Atlantic, she was 

 surrounded by a perfect swarm of Deiopeia pulcJiella, L., many 

 of which came on board ; her position at the time was 260 

 miles W.S.W. of the barren islet of St. Paul, 440 miles N.E. 

 of the nearest point of the American coast, and 960 miles 

 S.W. of the nearest land where this species is known to occur, 

 the Cape de Verde Islands. We have also many other similar 

 records extending over various parts of the world, and up to 

 our very shores ; for instance, Vanessa cardui, L., and 

 Diadema bolina^ Bdv., have been taken on board ship 200 

 miles off Cape de Verde Islands {Entom. iii. 326) ; and 

 Acerontia atropos, L., flew on to the ship " Cameron " when 

 passing the same place {E.M.M. xiv. 185). This latter 

 species has also been observed between Algiers and Gibraltar, 

 at the mouth of the English Channel {E.M.M. xviii. 295) ; 

 off the Irish coast some twenty-five miles out at sea {E.M.M. 

 X. 300) ; seven miles off Harwich, etc. {E.M.M. xii. 271), 

 Macroglossa stellatarum, L., was seen daily flying about a 

 steamer between Malta and Gibraltar, and again two days 

 before reaching Plymouth {E.M.M. xviii. 295) ; Pieris 



