1S6 GENERAL PHYSIOLOGY 



are produced in germ than as adults would find sufficient vital 

 conditions. To cite a striking example, it has been computed 

 that, if of the several million eggs that a sturgeon lays only one 

 million should develop into females and reproduce to an equal 

 extent, the third generation would find no room upon the surface 

 of the earth, while the fourth generation could produce a quantity 

 of eggs greater than the volume of the earth ! But this re- 

 markable condition is illusory, for only a very limited number of 

 individuals can find the proper conditions for their existence, all 

 others perish. But in this partly passive, partly active struggle 

 for the means of existence it is not the chance individuals that 

 perish, but almost exclusively those that can maintain the 

 struggle less long, that are less adapted to the given conditions. 

 On the other hand, those that are strongest, most powerful, most 

 capable of life under the given conditions, will overcome in the 

 competition and alone survive. Thus there takes place a se- 

 lection of individuals most fitted for the given conditions of life ; 

 and since this selection, as in breeding, continues for many and 

 finally innumerable generations, while the selected individuals 

 reproduce their characteristics by hereditary transmission, a 

 gradual adaptation of individuals to their external conditions 

 comes about, the result or expression of which is the purpose- 

 fulness, reaching to the minutest details, of organisms in relation 

 to the conditions under which they live. If the external con- 

 ditions remain for a time unchanged, adaptation acts in a con- 

 servative sense ; if they change, whether locally and suddenly, or 

 generally and gradually, as in the development of the whole eairth's 

 surface, there occurs by selective adaptation in the struggle for 

 existence a proportionate variation of form. The test of the 

 correctness of this theory lies in the experiments of animal 

 breeders, which have gone so far, especially in England, that by 

 artificial selection toward definite aims in the course of a few 

 years new vaiieties of domestic animals, especially pigeons, can be 

 supplied to order, having these or those desired qualities. Here the 

 artificial selection of the breeder plays the rSle of natural selection 

 which in free nature consummates the struggle for existence. 



Darwin's theory affords a comjDrehensive and consistent picture 

 of the origin of form-changes in living substance from the 

 simplest species that previously inhabited the surface of the earth 

 down to present organisms. If the effects of the few agents that 

 determine form are recognised, it is easy to understand naturally 

 the phylogenetic development of plants and , animals from the 

 unicellular protists, on the one side through cryptogams and 

 monocotyledons to the highly developed flowering-plants, and on the 

 other side through the coelenterates and worms to the highly 

 developed aithropods and vertebrates. 



