352 ME. Q. J. EOMAKJES ON" PHYSIOLOGICAL SELECTION. 



species ; 2nd, that the number of un-useful variations taking 

 place in all species is incalculable ; and 3rd, that the reproductive 

 system is so especially variable, both intrinsically and in response 

 to changed conditions of life, that increase of sterility must often 

 arise as a variation under nature. 



I have now fully, if not tediously, prepared the way for ex- 

 plaining the suggestion which I have to make. From what has 

 been said it may be concluded that all the multitude of individual 

 variations perpetually occurring in every species become re- 

 absorbed in the specific type by intercrossing, unless the varia- 

 tions happen to be either useful, to take place in isolation, or by 

 way of what Mr. Spencer calls " direct equilibration," such as 

 use, disuse, and so forth. It has also been shown that any 

 variations in the reproductive system which take place in the 

 direction of increased sterility must likewise tend to become 

 extinguished. But now it must be added that there is one such 

 variation in the reproductive system to which this remark does 

 not apply. For if the variation be such that the reproductive 

 system, while showing some degree of sterility with the parent 

 form, continues to be fertile within the limits of the varietal 

 form, in this case the variation would neither be swamped by 

 intercrossing, nor would it die out on account of sterility. On 

 the contrary, the variation would be perpetuated with more 

 certainty than could a variation of any other kind. For, in 

 virtue of increased sterility with the parent form, the variation 

 would not be exposed to extinction by intercrossing ; while, in 

 virtue of continued fertility within the varietal form, the varia- 

 tion would perpetuate itself by heredity, just as in the case of 

 variations generally when not re-absorbed by intercrossing. To 

 make my meaning perfectly clear I will use an illustration. 



Suppose the variation in the reproductive system is such that 

 the season of flowering or of pairing becomes either advanced or 

 retarded. Whether this variation be, as we say, spontaneous, or 

 due to any change of food, habitat, climate, etc., does not signify. 

 The only point we need here attend to is that some individuals, 

 living on the same geographical area as the rest of their species, 

 have varied in their reproductive systems, so that they can only 

 propagate with each other. They are thus perfectly fertile inter 

 se, while absolutely sterile with all the other members of their 

 species. This particular variation being communicated by inheri- 

 tance to their progeny, there would soon arise on the same area. 



