342 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



discoveries in-breeding is seen in a new aspect. The mating 

 together of closely related individuals derives both its ad- 

 vantages and disadvantages from the probable occurrence in 

 both parents of similar unit characters. Their offspring 

 will thus get a double portion of each common character. If 

 it is a good trait advantage will come of the mating, if a bad 

 trait disadvantage. Thus to accent and fix quickly a good 

 trait close breeding will be useful; to remove a bad trait out- 

 breeding will be necessary. In addition out-breeding practi- 

 cally always adds vigor to the offspring. But in many cases 

 of in-breeding vigor is not lost until after several or many in- 

 bred generations have been produced. 



Selection, Natural and Artificial. Natural selection, which 

 is a phrase summing up the presumable result of the interaction 

 on animals and plants of several distinct natural causes and 

 conditions, has been generally considered since Darwin's time 

 to be the principal factor in evolution. It is the factor chiefly 

 relied on by Darwin in his explanation of evolution, and its 

 formulation is Darwin's principal original contribution to 

 evolutionary discussion. 



Natural selection depends upon, first, the geometrical ratio 

 of reproduction, resulting in the production of more individuals 

 than there is food or space for, this overproduction of living 

 creatures necessitating a "struggle for existence" among them; 

 second, the universal occurrence of variations, small or large, 

 among all the myriads born; third, the presumption that some 

 of these variations will give advantages to certain individuals 

 in this struggle, resulting in their success and growth to 

 maturity and production of young; fourth, the inheritance 

 by the young of the advantageous variations of their parents, 

 and their handing on of them to their own succeeding genera- 

 tions thus constituting a new race or species characterized 

 by the original new variations and succeeding new and better 

 ones. Thus there is a "natural selection" of individuals 

 within a species, and also a "natural selection" of specially 

 favored species in their competition with other species. The 

 whole steadily acting combination of conditions results in a 

 constant movement from one kind of animal or plant type to 



