VARIATION IN SIZE. 
,S4 
either more vigorous while in progres.s, or the growth season longer 
continued, the result in each case being the development of a larger 
animal and shell. As this optimnm environment probably varies 
for each .species, it follows that where these optimnm conditions are 
most nearly approached, the species most favonrably affected will 
develop the largest ami most vigorous mollu.sks, and divergence 
from this optimum would result in a i)roportionately smaller animal 
and shell. 
Mqnahilifj'and suitability of temperatnre during tlie growth period, 
which is essential to the development of the finest mollusks, is not 
exclusively dependent ujion latitude, being largely intluenced by the 
coidiguration of the land, the vicinity of the sea, and the prevailing 
direction of winds and currents. Thus the British Isles, and more 
especially Ireland, have -owing to their insular position and other 
causes - an eipiahle and moist climate, and do not suffer from the 
great heat in summer and the intense cold in winter that characterize 
the great continental areas in the .same latitude, each of which 
imiioses a check upon growth if occurring during the growth season. 
])r. Jeffreys has asserted that 
northern specimens generally 
attain a larger size than the 
same sjiecies in the south, and 
(piotcs Draiiarnand’s remarks 
.on the comi»arative size of the 
shells of north and south France 
in snp[)ort of his opinion; hut 
this result can only occur with 
such species as are most suitably 
circumstanced in such districts, 
and this is not merely a question 
of a larger food supply being ohfainalde, 
owing to the greater i)ancity of species 
and individuals in northern districts, as 
Dr. Jeffreys has supposed, but is depen- 
dent, amongst other things, upon the 
existence of a temperatnre at which 
assimilation can he actively carried on. 
(S})ecimens of IfeJix Kuprrxit from our 
own country are far inferior in size to 
Fig. 101 . — Helix nspc7‘sa var. 
minor Picard, 
Cardiir, 
Collected by Mr. F. \V. Wotlon, 
Showin.-T the efTects of some un- 
favourable conditions of the environ- 
ment. 
