128 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
for the belief that they must needs doso. The analogy of 
a machine that, sooner or later, must be brought to a 
standstill by the wear and tear of its parts, does not 
hold, inasmuch as the animal mechanism is continually 
renewed and repaired ; and, though it is true that indi- 
vidual components of the body are constantly dying, yet 
their places are taken by vigorous successors. A city 
remains, notwithstanding the constant death-rate of its 
inhabitants ; and such an organism as a crayfish is only 
a corporate unity, made up of innumerable partially 
independent individualities. 
Whatever might be the longevity of crayfishes under 
imaginable perfect conditions, the fact that, notwithstand- 
ing the great number of eggs they produce, their number 
remains pretty much the same in a given district, if 
we take the average of a period of years, shows that 
about as many die as are born; and that, without the 
process of reproduction, the species would soon come to 
an end. 
There are many examples among members of the group 
of Crustacea to which the crayfish belongs, of animals which 
produce young from internally developed germs, as some 
plants throw off bulbs which are capable of reproducing 
the parent stock; such is the case, for example, with the 
common water flea (Daphnia). But nothing of this kind 
has been observed in the crayfish; in which, as in the 
higher animals, the reproduction of the species is de- 
pendent upon the combination of two kinds of living 
