VII ON THE INFERTILITY OF CROSSES 183 



on the most favourable estimate, the physiological variety can 

 never exceed 12,000 to the 88,000 of the normal form of the 

 species, as shown by the foUoAving table : — 



1st Year. 10,000 of physiological variety to 90,000 of normal variety. 

 2d „ 1,220 + 10,000 again produced. 



3d „ 16 + 1,220 + 10,000 do. = 11,236 



4th „ + 16 + 1,220 + 10,000 do. = 11,236 



5th „ 0+16+ 1,220 + 10,000 = 11,236 



and so on for any number of generations. 



In the preceding discussion we have given the theory the 

 advantage of the large proportion of 10 per cent of this very 

 exceptional variety arising in its midst year by year, and we 

 have seen that, even under these favourable conditions, it is 

 unable to increase its numbers much above its starting-point, 

 and that it remains Avholly dependent on the continued 

 renewal of the variety for its existence beyond a few years. 

 It appears, then, that this form of inter -specific sterility 

 cannot be increased by natural or any other known form of 

 selection, but that it contains within itself its own principle 

 of destruction. If it is proposed to get over the difficulty by 

 postulating a larger percentage of the variety annually arising 

 within the species, Ave shall not affect the laAV of decrease until 

 Ave approach equality in the numbers of the tAvo varieties. 

 But Avith any such increase of the physiological A^ariety the 

 species itself would inevitably suffer by the large propor- 

 tion of sterile unions in its midst, and Avould thus be at a 

 great disadA'antage in competition Avith other species Avhich 

 Avere fertile throughout. Thus, natural selection will always 

 tend to Aveed out any species AAath too great a tendency to 

 sterility among its oAvn members, and Avill therefore prevent 

 such sterility from becoming the general characteristic of vary- 

 ing species, Avhich this theory demands should be the case. 



On the Avhole, then, it appears clear that no form of 

 infertility or sterility between the indiAdduals of a species, 

 can be increased by natural selection unless correlated A\dth 

 some iiseful variation, AAdiile all infertility not so correlated 

 has a constant tendency to eftect its oavu elimination. But 

 the opposite property, fertility, is of A'ital importance to every 

 species, and gives the offspring of the individuals AA'hich 

 possess it, in consequence of their sui)erior numbers, a greater 



