VII ox 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 follo^ving 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 „ 0+ 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 wholly 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 knoAvn 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, we shall not affect the law of decrease until 

 we approach equality in the numbers of the tw^o varieties. 

 But 'with any such increase of the physiological variety the 

 species itself woukV inevitably suffer by the large projDor- 

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

 great disadvantage in competition "s^ith other species which 

 were fertile throughout. Thus, natural selection will always 

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

 sterility among its own members, and will therefore prevent 

 such sterility from becoming the general characteristic of vary- 

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



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

 infertility or sterility between the individuals of a species, 

 can be increased by natural selection unless correlated with 

 some useful variation, while all infertility not so correlated 

 has a constant tendency to effect its own elimination. But 

 the opposite property, fertility, is of vital importance to every 

 species, and gives the offspring of the individuals which 

 possess it, in consequence of their superior numbers, a greater 



