1886.] Zs Littorina litorea Introduced or Indigenous ? 939 
But again, what is the meaning of its wonderfully rapid spread, 
and why, if it existed in Nova Scotia previous to say 1850, did it ` 
not begin to spread before? Its spreading as rapidly as it has, 
shows that it was only waiting for the opportunity to take advan- 
tage of it, but why, if it is indigenous, did it not begin to spread 
Sooner? Surely the same causes which have carried it south 
since 1850 were in operation before. If they were natural, such 
as currents, etc., they certainly have been present substantially 
unchanged for centuries. Professor Verrill suggests that it may 
have existed formerly in Nova Scotia, but have “become more 
diffused in recent times by commerce.” But surely there was 
commerce between Nova Scotia and New England before 1868 
(in which year it was first reported from Maine), and enough of 
it to satisfy the most exacting demands of this theory. In all 
Probability the rapid diffusion of the shell since 185 7 is ina 
measure due to both of these causes, but the fact that they did 
not have a like effect before, seems very strongly to show that 
the shell was not in Nova Scotia for them to spread. The waters 
which bathe the Atlantic coast of Nova Scotia are carried by the 
strong Fundy tides across to the New Brunswick and Maine 
Coasts, and if currents had anything to do with carrying L. palli- 
ata from one continent to Greenland and thence to the other, it 
should have carried the free-swimming embryos of its ally, Z. 
litorea, from the Nova Scotia to the New England coast. 
But granting again for a moment that Z. /torea has existed in 
Nova Scotia for an indefinitely long time as an indigenous species, 
we have it existing under conditions very different from those in 
Which it thrives in England, having, as has been shown, no con- 
nection with the latter, and yet retaining its specific identity. It 
'S possible for a species to keep its identity in widely separated 
localities, where the conditions of life are not precisely the same, 
only by a continuous intercourse between the different localities. 
This is in all probability the case with Z palliata, for we find it 
ranging freely around the North Atlantic in England, Greenland, 
_“@brador, Acadia and New England, and the agencies which 
_ “tried it from one land to the other have in all probability been 
& 19 operation ever since. But with Z. Žtorea the case is differ- 
_ ‘Snt; if it existed in Nova Scotia it must have been cut off from _ 
f= Communication with England, and that it should retain j 
*We have found no list of the shells of Iceland. 
