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 following 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 
6th „ 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 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, we shall not affect the la w of decrease until 
we approach equality in the numbers of the two varieties. 
But with any such increase of the physiological variety the 
species itself would inevitably suffer by the large propor¬ 
tion of sterile unions in its midst, and would thus be at a 
great disadvantage in competition with other species which 
were fertile throughout. Thus, natural selection will always 
tend to weed 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 
