River Temperature. 209 
from day to day is of the same amount. On the other hand, 
in a pond during ordinary summer weather a molluse has 
the choice of a great variety of temperatures. It may prefer 
the cool bottom-waters, with a temperature, for instance, of 
65°; or it may creep beneath the surface film, where the 
temperature is 10° higher, viz., 75°; or it may estivate in 
water 2 or 3 inches deep at the margins, where the 
temperature is between 80° and 85°. Nor is this all. At 
the bottom, as above remarked, it experiences not only a 
temperature near that of a river, but a temperature with a 
similar limited daily range of a degree or two ; and, as far as 
the thermal conditions are concerned, we might expect that 
the molluscan fauna of a free-flowing river would find its 
counterpart in the molluscan life of the bottom of a pond. 
If the creature ascended to the surface, it would experience 
a daily range of temperature of 6° or 7°; whilst in the 
marginal shallows it would be exposed to a fluctuation of 
temperature of as much as 15° or 20° in the day and night. 
It is, therefore, legitimate to suppose that these variations in 
a pond’s temperature in summer, which exist, though usually 
to a less marked extent in spring and autumn, are reflected 
in the divergent habits of river and pond molluses. Marked 
differences should be found between them as regards their 
estivation, the deposition of their spawn, the hatching of 
the eggs, the behaviour of their fry, and in the age and size 
adults attain. 
The differences in the thermal behaviour of ponds and 
rivers will affect plant-life in a multitude of ways. In Part 
I, pp. 295, 296, I have endeavoured to show how the absence 
of surface-heating in a river, and its presence in a pond, may 
be reflected in the budding, flowering, and fruiting periods 
of aquatic plants. It is conceivable that they may also 
influence the form of a plant, as in the case of the various 
forms of Callitriche aquatica and Ranunculus aquatilis. 
Those kinds of the first-named plant that possess submerged 
linear leaves and floating rosettes of obovate leaves behave 
differently in winter in a pond and in a perennial spring, 
In a pond the floating rosettes die and disappear in winter, 
and others are produced in April, the plant in its winter 
