THE STRUGGLE FOR LIFE. 139 
germs are exposed in their growth to maturity, the more 
numerously are they produced. This statement applies to 
both kingdoms of Nature—the Animal and the Vegetable. 
For instance, it is calculated that the roe of a single cod fish 
may contain more than 9,000,000 eggs at the same time; and 
again, the progeny of a single female Aphis, such as the rose- 
blight, would, in the course of her life of about six weeks, 
if ail could be preserved and provided with sufficient food, 
amount to many billions. Again, the number originating 
from a single Bacterium might in a day exceed 161% millions. 
If from any cause a species multiplies unduly, its natural 
enemies also begin to increase in number, since abundance of 
food allows more young to be reared, and so nature begins to 
readjust itself. Many of you will remember reading of the 
plague of Voles, which devastated the lowland pastures of 
Scotland a few years ago. It was soon evident that unusual 
numbers of their natural enemy, the Short-eared Owl, were to 
be found there, and these bred far more freely than usual. 
Now that the number of voles has been reduced, the owls 
are dying of starvation. ‘lhe increase of the Canadian rabbit, 
which takes place periodically to such an alarming extent 
that it threatens destruction to all the crops, is always accom- 
panied by a great increase in the numbers of the lynx and 
other carnivorous animals which prey upon it; yet after a 
time this does not suffice to abate the nuisance, and then 
they commence to die in myriads, and their carcases are found 
in all directions, so that it becomes difficult to meet with a 
single living specimen. This destruction, when the country 
is overstocked, is probably due to the ravages of a Bacterium, 
as Dr. Loftler has successfully treated a similar visitation of 
voles in the plains of Thessaly, by poisoning a number of in- 
dividuals with the Bacterium of the Typhus of the mouse, 
their neighbours, having cannibal propensities, ate their dead 
friends, and so spread the plague among themselves. So far, 
I have only spoken of cases where the forces of nature have 
