282 ANIMAL STUDIES 



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- 

 eel laj^^^s^jaj^lA^OjOOO^gg^ 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. 

 Linnaeus once said that " three flies would devour a dead 

 horse as quickly as a lion." Equally soon would it be de- 

 voured 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- 



