THE STRUGGLE FOR EXISTENCE 115 
the death rate, and those which “live beyond their means ” 
must sooner or later disappear. One of the most abundant 
of birds is the fulmar petrel, which lays but one egg yearly. 
It has but few enemies, and this low rate of increase suf- 
fices to cover the seas within its range with petrels. 
It is difficult to realize the inordinate numbers in which 
each species would exist were it not for the checks produced 
by the presence of other animals. Certain Protozoa at their 
normal rate of increase, if none were devoured or destroyed, 
might fill the entire ocean in about a week. The conger- 
ecl lays, it is said, 15,000,000 eggs. If each egg grew 
up to maturity and reproduced itself in the same way in 
less than ten years the sea would be solidly full of conger- 
eels. If the eggs of a common house-fly should develop, and 
each of its progeny should find the food and temperature it 
needed, with no loss and no destruction, the people of a city in 
which this might happen could not get away soon enough to 
escape suffocation from a plague of flies. Whenever any in- 
sect is able to develop a large percentage of the eggs laid, it 
becomes at once a plague. Thus originate plagues of grass- 
hoppers, locusts, and caterpillars. But the crowd of life is 
such that no great danger exists. The scavenger destroys 
the decaying flesh where the fly would lay its eggs. Minute 
creatures, insects, bacteria, Protozoa are parasitic within 
the larva and kill it. Millions of flies perish for want of 
food. Millions more are destroyed by insectivorous birds, 
and millions are slain by parasites. The final result is that 
from year to year the number of flies does not increase. 
Linnezus once said that “three flies would devour a dead 
horse as quickly as a lion.” Equally soon would it be de- 
youred by three bacteria, for the decay of the horse is due 
to the decomposition of its flesh by these microscopic plants 
which feed upon it. ‘“ Even slow-breeding man,” says Dar- 
win, “has doubled in twenty-five years. At this rate in less 
than a thousand years there would literally not be standing 
room for his progeny. The elephant is reckoned the slow- 
