80 
VARIATION IN SIZE. 
other species, as Anculm fnriatUis, under similar conditions seem 
unable to do so, their shells being unusually delicate and thin. 
This dift’erence in the selective working of the tissues in different 
species is true also in a lesser degree amongst the individuals com- 
posing a species, which vary int(n- se in their power to utilize the 
shell-forming material presented, although the thickness of the shell 
would generally seem to be in inverse ratio to the hardiness of the 
animal, the most hardy species or genera, or those which withstand the 
most rigorous climates having only a thin external or internal shell, 
as though the shell-forming energy of the creature was diverted to 
strengthen more vital processes. 
Size is not only influenced by the obvious causes of the abundance 
or scarcity of suitable and nutritious food, the result of which would 
be the proiluction of a larger or a more diminutive animal and shell 
than would be developed under ordinary conditions, but is also in a 
large degree dependent upon temperature and other circumstances. 
The researches of Semper on the phenomena of growth, upon which 
size is dependent, have shown that in Limuau stagnalis the size 
attained by the shell is capable of correlation with the temperature 
and amount of the inhabited water, as assimilation and growth 
ecpuilly ceased if the degree of warmth exceeded 00° P. or fell below 
53° P., the fullest vigour being enjoyed and the largest size attained 
when the temperature ranged between 08° P. and 77° P. 
The results of these researches are of great interest, and are more 
or le.ss applicable to other specie.s, as demonstrating some of the con- 
ditions governing growth — and therefore size — in mollusca generally, 
and clearly establish that 
the volume or amount of 
water allowed to eachinollusk 
is so decisive in its effect 
upon growth that in the space 
of six days the difference in 
the size of the shells of those 
in a large and those in a small 
body of water becomes apparent ; the smaller the amount of water 
per individual the smaller the shell and vice irrsft If the volume 
of water be less than 5,000 cubic centimetres per individual, a 
dwarfing influence is perceptible, the greatest differential effect being 
P'lG. 173. — Volume-curve of Liuitura stagnalis^ 
showing the effects of various volumes of water upon 
the size attained in a defmite period of G.3 days (after 
Semper). 
