198 | February, 
very recent one, the actual length of time which has since passed is 
so great, that I presume few geologists would venture to compute it 
even in thousands of years. And although there has evidently been 
considerable oscillation of the land during the subsequent period, the 
deposits of gravel, &ec., in different parts of the fen valley, indicate 
that fresh water agencies were at work, and that the sea had not the 
same action on the old coast line, since the later Post-Glacial period. 
This view is confirmed by the absence of marine shells in these deposits, 
while the immense lapse of time is further shown by the presence of 
an abundance of a fresh-water shell (Cyrena flumenalis) imbedded in: 
them, although the species has now totally disappeared from the seas: 
of the north of Europe, and is not known to occur nearer than the) 
mouth of the Nile. | 
The occurrence of these coast sand-hill insects on this ancient: 
sea-shore is therefore a circumstance of considerable interest, particu. 
larly as they appear to be by no means rare there (indeed, Gelechia) 
desertelia is most abundant), and the question naturally arises how’ 
they reached so congenial a spot. | 
The intermediate answer to be expected is “by migration,” and! 
theories in plenty instantly crop up of chance specimens carried the: 
whole distance by winds. A moment’s consideration of the habits and! 
structure of most of these species, however, shows such a solution to. 
be utterly untenable. Whatever the strong Mamestra albicolon 
might do, it is very unlikely that it would allow itself to be carried’ 
by any wind across two miles of country, far less twenty ; while we all 
know how carefully the weak <Anerastia lotella and the lively little 
Gelechie avoid exposing themselves to a breeze, and that they will 
never move on the wing except in calm weather, or good shelter. 
Moreover, the “blown across” theory can only hold good over a 
level surface like the sea, and in the case of strong insects, such as: 
butterflies, which, provided the sun be shining, will brave a consider-. 
able amount of wind, but over a country covered with scattered trees, 
with occasional hills and other inequalities of surface, the disturbing 
currents caused thereby would inevitably precipitate all such matters 
to the ground, or enable them to reach it. 
Ordinary migration of species cannot be accepted as a solution of 
the difficulty, since, as I have already stated, the intermediate country 
is utterly unsuitable for the existence of these sand-loving insects, 
It consists of fen, wood, heath and cultivated country. The heaths are} 
sandy tracts as a matter of course, and I am informed that there are 
