182 
DARWINISM 
CHAP. 
form or colour, or with inherent peculiarities of likes or 
dislikes leading to any choice as to the pairing of the two sets 
of individuals. We have now to inquire, What would he the 
result 1 
Taking, first, the 10,000 pairs of the physiological or 
abnormal variety, we find that each male of these might 
pair with any one of the whole 100,000 of the opposite 
sex. If, therefore, there was nothing to limit their choice 
to particular individuals of either variety, the probabilities 
are that 9000 of them would pair with the opposite variety, 
and oidy 1000 with their own variety—that is, that 9000 
would form sterile unions, and only one thousand would form 
fertile unions. 
Taking, next, the 90,000 normal individuals of either sex, 
we find, that each male of these has also a choice of 100,000 
to pair with. The probabilities are, therefore, that nine- 
tenths of them—that is, 81,000—would pair with their 
normal fellows, while 9000 would pair with the opposite 
abnormal variety forming the above-mentioned sterile unions. 
Now, as the number of individuals forming a species 
remains constant, generally speaking, from year to year, we 
shall have next year also 100,000 pairs, of which the two 
physiological varieties will be in the proportion of eighty-one 
to one, or 98,780 pairs of the normal variety to 1220 1 of 
the abnormal, that being the proportion of the fertile unions 
of each. In this year we shall find, by the same rule of 
probabilities, that only 15 males of the abnormal variety will 
pair with their like and be fertile, the remaining 1205 forming 
sterile unions with some of the normal variety. The follow¬ 
ing year the total 100,000 pairs will consist of 99,984 of the 
normal, and only 16 of the abnormal variety; and the prob¬ 
abilities, of course, are, that the whole of these latter will 
pair with some of the enormous preponderance of normal 
individuals, and, their unions being sterile, the physiological 
variety will become extinct in the third year. 
If now in the second and each succeeding year a similar 
proportion as at first (10 per cent) of the physiological variety 
is produced afresh from the ranks of the normal variety, the 
same rate of diminution will go on, and it will be found that, 
1 The exact number is 1219 '51, but the fractious are omitted for clearness. 
