THE METHOD OF EVOLUTION 327 



alone, but has influenced almost every branch of science. In its 

 broader features it is accepted by every biologist, although there 

 are many details still to be worked out. 



Following is an outline of the chief factors assigned by Darwin 

 to account for the development of new species from common 

 ancestry. 



1. Over-production of individuals. 



2. Struggle for existence. 



3. Variation among individuals. 



4. Survival of the fittest. 



5. Inheritance of favorable characteristics. 



6. New forms better adapted to survive are thus " naturally 

 selected " as new species. 



Darwin spent over twenty years of strenuous toil and study, 

 accumulating facts upon which to base his theory. Many able 

 men have since devoted their lives to the same end, but we can 

 here only briefly review the argument, following the outline given 

 above. 



Over Production. A fern plant may produce fifty million spores 

 per year. If all matured they would completely cover North 

 America the second year. A mustard plant produces 730,000 

 seeds annually, which if all matured, would occupy two thousand 

 times all the land surface of the earth, in two years. The common 

 dandelion would accomplish the same in about ten years. 



The English sparrow lays six eggs at a time and breeds four 

 times a year; if all survived there would be no room for any other 

 birds in the course of a decade. The codfish produces over a 

 million eggs per year; if all survived this would fill the Atlantic 

 solidly with fish, in about five years. 



Most amazing of all is the rapidity of reproduction in bacteria 

 and protozoa. One of the latter, if it reproduced unchecked, 

 would make a solid mass of these microscopic animals as large 

 as the sun, in thirty-eight days. 



Struggle for Existence. We know there is no such actual in- 

 crease; in fact the number of various forms changes but little. 



