Phylogeny of the Pierinee. 325 
With regard to the first, there is little difficulty in 
supposing the passage to have been effected by land 
either now or formerly existing, the Ethiopian Region 
having been in this, as in so many other instances, first 
entered from the north. But it is not easy with our 
present knowledge to imagine an overland passage for 
these butterflies from the Oriental or Ethiopian Region 
to the Neotropical. The northward route, which we saw 
to be the one probably adopted by the ancestors of the 
Chilian Phulias and Tatochilas in spreading from Central 
Asia, is excluded in the present instance by the entire 
absence of any trace of such a passage from both 
the Palearctic and Nearctic Regions; and although a 
transit by way of a formerly existing “ Antarctica” is 
conceivable, it would seem more likely that the crossing 
from east to west was effected in the region of the tropics. 
After all, however, the difficulty of supposing an Atlantic 
sea-passage is not overwhelmingly great. The unusual 
facilities possessed by insects for crossing large extents of 
sea have been remarked by many writers,* and among 
insects the butterflies of the Pierine group are especially 
given to migration for great distances in countless hordes. + 
It is worth noting that im the case of each of the other 
three chief Pierine genera whose present distribution 
seems to have involved one or more long sea-passages, 7.e., 
Terias, Colias, and Callidryas, special observations exist 
of their migratory propensities. In 1874 a large swarm 
of Terias lisa reached the Bermudas from the American 
continent ;{ the swarm of butterflies described in a well- 
known passage|| by Mr. Darwin consisted chiefly of a 
species of Colias; while descriptions of the migratory 
flight of Catopsilia and Callidryas are numerous, 
among the most striking pieces of testimony being that 
* See especially Wallace’s “Geographical Distribution,” 1876, 
vol. i., p. 832; and the same author’s “ Darwinism,” 1889, p. 359, etc. 
+ Trimen, Trans, Ent. Soc. Lond., 1870, p. 382 ; “ South-African 
Butterflies,’ 1887, vol. ui., p. 32. Moore, ‘‘ Lepidoptera of 
Ceylon,” 1880, 81, p. 116. Distant, “ Rhopalocera Malayana,” 
1882—86, p. 285, etc. Mr. Trimen suggests that there is “an 
evident connection or relation between these wonderful migrations 
of certain species of Pieréne and the well-known habit of nearly 
all the members of the Sub-family of flying straight onward in 
one direction.” 
t “ Psyche,” Dec. 1875, p. 121. 
|| “ Voyage of the Beagle,” ed. 1860, p. 158. 
