1890 ON PHYSIOLOGICAL SELECTION 247 



The force of the female and length of fertile 

 period, &c., is enough and no more to bear enough 

 offspring under all these conditions to maintain her 

 representation in the species. (We use ' provide,' 

 ' maintain ' poetically.) 



She has no time to spare. She lives long enough 

 to replace herself in species and no longer. 



[In what follows] I say ' mare ' for short. At 

 present mares are not under natural conditions. And 

 no doubt in high feeding, &c., and protection pro- 

 duce many more ova than enough, and so on through 

 the series. In nature the ground is full stocked. 

 1 Enough and no more ' is the badge of all our 

 tribe. 



Take then a species of animals, a number of indi- 

 viduals, closely related, all alike, and freely interbreed- 

 ing and producing fertile and nearly similar offspring. 



Now suppose two varieties to arise by any means 

 known. Is there any things which will produce in- 

 sterility between these varieties without a new addi- 

 tional factor of segregation ? 



A mare of variety A might bear say twenty off- 

 spring of variety A. If she spends force in bearing 

 ten of variety A B, on average she will bear ten only 

 of A. 



The intermediate form is at a disadvantage : see 

 Wallace. 



The mare then has only half her representation 

 in the extreme variety A. (And the others are no 

 good to her for prolonging her seed, in the long run). 



