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 be the 

 result ? 



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 only 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 98,7 80 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 1 6 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 fractions are omitted for clearness. 



