EVOLUTION AS A NATURAL PROCESS 123 



figures enumerating the offspring of a single pair of 

 birds at the end of fifteen years, if again all individuals 

 lived complete and normal lives : at the end of the 

 time specified there would be more than two thousand 

 millions of descendants. The English sparrow has been 

 on this continent little more than fifty years ; it has 

 found the conditions in this country favorable be- 

 cause few natural enemies like those of its original home 

 have been met, and as a consequence it has multi- 

 plied at an astounding rate so as to invade nearly all 

 parts of North America, driving out many species 

 of song birds before it. About twenty years ago 

 David Starr Jordan wrote that if the English sparrow 

 continued to multiply at the natural rate of that time, 

 in twenty years more there would be one sparrow to 

 every square inch of the state of Indiana ; but of course 

 nature has seen to it that this result has not come 

 about. A single conger-eel may produce fifteen million 

 eggs in a single season, and if this natural rate of 

 increase were unchecked, the ocean would be filled 

 solid with conger-eels in a few years. Sometimes a 

 single tapeworm, parasitic in the human body, will 

 produce three hundred million embryos; the fact 

 that this animal is relatively rare diverts our attention 

 from the alarming fertility of the species and the ex- 

 cessive rate of its natural increase. Perhaps the most 

 amazing figures are those established by the students 

 of bacteria and other micro-organisms. Many kinds 

 of these primitive creatures are known where the 

 descendants of a single individual will number sixteen 

 to seventeen millions after twenty-four hours of develop- 

 ment under ordinarily favorable conditions. Though 



