NATURAL SELECTION; SEXUAL SELECTION 59 



metric ratio, but as it multiplies it finds the world already 

 crowded with other multiplying species. A single pair of any 

 species whatsoever, if not checked by adverse conditions, would 

 soon fill the whole earth with its progeny. 



An annual plant producing two seeds only would have 

 1,048,576 descendants in twenty-one years, if each seed sprouted 

 and matured. But most plants produce hundreds or thousands 

 of seeds. The ratio of increase is a matter of minor imj)ortance. 

 It is the ratio of increase above loss whicli determines tlie fate 

 of species. Those species increase in numbers in wliicli the 

 gain exceeds the rate of destruction througli the influence of 

 other species or the adverse conditions of life. Where few 

 enemies exist the ratio of increase need not be large. One of 

 the most abundant of birds is the fulmar petrel of the mid- 

 Pacific. It lays but one egg yearly, but it has few enemies and 

 the low rate of increase suffices to cover the sea with fulmars 

 within the region it inhabits. 



It is not easy to realize the inordinate numbers any species 

 would attain were it not for the checks produced by the presence 

 of the activity of other organisms. Certain protozoa, at their 

 normal rate of increase — if none were devoured or destroyed — 

 might fill the entire ocean within a very short time. It is said 

 that the conger eel lays 15,000,000 eggs yearly. If each hatched 

 and the conger grew to maturity, in a few years there would be 

 no room for any other kind of fish in the sea. The codfish has 

 been known to produce 9,100,000 eggs each year. If each egg 

 were to develop, in ten years the sea would be solidly full of 

 codfish. 



The female quinnat salmon of the Columbia, Oncorhynchus 

 tchamytscha, ascends the river at the age of about four years, 

 and lays 4,000 eggs, after which she dies. Half these eggs 

 develop into males. If each female egg came to maturity, we 

 should have at the end of fifty years 8,000,000,000,000,000,- 

 000,000,000,000,000,000,000,000,000 female salmon and as 

 many males as the offspring of a single pair. It takes al^out 

 one hundred of these salmon to weigh a ton. Could all these 

 fishes develop, in a very short time there would be no room 

 for them in all the I'ivers c^ the North, nor in all the waters of 

 the sea. 



If each egg of the conmion house fly slioiild develop and each 

 of the larva) should find the food :nd temperature it neech'd 



