Artificial and Natural Selection 799 



knowledged as the first step in the way 

 of selection, and is designated by the name 

 of variety-testing, applying the term variety to 

 all the subdivisions of systematic species indis- 

 criminately. In natural processes it bears the 

 title of survival of species. The fact that re- 

 cent types show large numbers, and in some in- 

 stances even hundreds of minor constant forms, 

 while the older genera are considerably reduced 

 in this respect, is commonly explained by the 

 assumption of extinction of species on a corre- 

 spondingly large scale. This extinction is con- 

 sidered to affect the unfit in a higher measure 

 than the fit. Consequently the former vanish, 

 often without leaving any trace of their exist- 

 ence, and only those that prove to be adapted 

 to the surrounding external conditions, resist 

 and survive. 



This selection exhibits far-reaching analo- 

 gies between the artificial and the natural proc- 

 esses, and is in both cases of the very highest 

 importance. In nature the dying out of unfit 

 mutations is the result of the great struggle for 

 life. In a previous lecture we have compared 

 its agency with that of a sieve. All elements 

 which are too small or too weak fall through, 

 and only those are preserved which resist the 

 sifting process. Eeduced in number they 

 thrive and multiply and are thus enabled to 



