354 MK. a. J. ROMANES ON PHYSIOLOGICAL SELECTION. 



The principle thus briefly sketched in some respects resembles 

 and in other respects differs from the principle of natural selec- 

 tion, or survival of the fittest. For the sake of convenience, 

 therefore, and in order to preserve analogies with already existing 

 terms, I will call this principle Physiological Selection, or Segre- 

 gation of the Fit. 



Aegitments a peioei. 



Before stating the evidence which I have been able to collect 

 of the operation of this principle, it is desirable that I should 

 make one or two general remarks upon the conditions under 

 which alone this evidence can be presented. 



First, let it be observed that if this particular kind of variation 

 ever takes place at all, we are not concerned either with its 

 causes or with its degrees. Not with its causes, because in this 

 respect the theory of physiological selection is in just the same 

 position as that of natural selection ; it is enough for both that 

 the needful variations are provided, without it being incumbent 

 on either to explain the causes which underlie the variations. 

 Nor is the theory of physiological selection concerned with the 

 degrees of sterility which may in any particular cases have been 

 initially supplied. For, whether the degree of sterility with the 

 parent form is originally great or small, the result of it in the 

 long run will be the same ; the only difference will be that in the 

 latter case a greater number of generations would be required in 

 order to separate the varietal from the parent form, as a little 

 thought will be enough to show *. 



Next, let it be observed that, from the nature of the case, we 

 cannot expect to meet with much direct evidence of physiological 

 selection yielded by our domesticated varieties. For, first, it has 

 never been the object of breeders or horticulturists to go back to the 

 wild stocks, and therefore observations on this point are wanting ; 

 second, breeders and horticulturists keep their strains separate, 

 and many kinds of variation are preserved other than those 



* Suppose that, on an average, a cross between the parent and the variety 

 were to yield a progeny of 2, while a cross between two individuals of the new 

 variety were to yield a progeny of 3. In this ease there is but a very small 

 degree of sterility towards the parent form ; yet if ligured out it will be found — 

 supposing this degree of sterility to be inherited by the pure-bred varieties — 

 abundantly sufficient to ensure multiplication of the varietal type, without 

 danger of this type being swamped by the parental. 



