288 Natural Selection [CH. 



doubtful, there was nothing to indicate how or when it was 

 determined. We now see that the discontinuous variations 

 are in the main the outward manifestations of the presence 

 or absence of corresponding Mendelian factors, and we 

 recognize that the unity of those factors is a consequence 

 of the mode in which they are treated by the cell-divisions 

 of gametogenesis. With the discovery of these factors 

 precise analytical treatment can at length be applied to the 

 problem of Evolution. 



3. Mendelism and Natural Selection. 



Knowledge of the physiology of heredity thus abolishes 

 an old difficulty often admitted to be an obstacle in the 

 way of any affirmation of Evolution by the process of natural 

 variation. The notion that a character once appearing in 

 an individual is in danger of obliteration by the intercrossing 

 of that individual with others lacking that character proves 

 to be unreal ; because in so far as the character depends 

 on factors which segregate, no obliteration takes place. 

 The factors are permanent by virtue of their own properties, 

 and their permanence is not affected by crossing. 



If the acquisition of a new factor or the loss of an old 

 one is so damaging as sensibly to impair the chances of 

 life of the variety thus constituted, that variety must surely 

 be extinguished. On the contrary, the addition of a new 

 factor contributing sensibly to the success of its possessor, 

 or the omission of a detrimental element, will aid in the 

 preservation of those which exhibit that variation. 



Mendelism, though aiding us greatly in showing how 

 the diversity of species and varieties may arise and be 

 maintained, provides no fresh clue to the problem of Adapta- 

 tion ; except in so far as it is easier to believe that a 

 definite, integral change in attributes can make a per- 

 ceptible difference to the prospect of success than that an 

 indefinite and impalpable change should entail such conse- 

 quences. Definite variational changes are being continually 

 offered, each giving an opportunity to natural or to artificial 

 selection, and we need not hesitate to declare that of such 

 materials the diversity of nature has been compiled. If 

 anywhere in such a province as evolutionary science certainty 



