1897] REPRODUCTIVE DIVERGENCE 183 



ductive divergence" may come into operation. Let 100 individuals 

 breeding with similar sized individuals, give birth to 120 offspring 

 of either sex instead of 100, whilst 100 individuals breeding with 

 moderately smaller or larger individuals (i.e., M and m breeding with 

 s and S or I and L) give birth to, on an average, only 95 offspring, 

 and 100 individuals breeding with considerably smaller or larger 

 individuals (i.e., S or L breeding with / or s) give birth to only 80 

 offspring of either sex. Then it will be found that the 900 males 

 and females breeding together will give birth to the following : — 

 120 Ss, 190 Sm, 280 Mm, 190 Ml, 120 LI, 



That is to say, whilst the largest and smallest individuals have 

 increased in numbers by 20 per cent., the medium sized ones have 

 decreased by 7 per cent, and the ones intermediate between these 

 by 5 per cent. The fact that the medium sized individuals have 

 decreased in number, in spite of the 100 M and m individuals which 

 breed together having produced 120 Mm, offspring, is of course due to 

 the fact that only 160 Mm individuals are produced by the crossing 

 of the 100 S and 100 L individuals with the 100 / and s. 



In a similar manner, in succeeding generations, the numbers of 

 individuals intermediate in size will gradually become smaller and 

 smaller, whilst those of the extreme ones will increase. But, it 

 may be said, even then the two varieties thus formed will not 

 differ in size to a greater extent than the extreme individuals in the 

 original 1800 taken. This is not the case. Thus supposing the 

 three groups of individuals were respectively on an average 65'5, 

 68*5 and 71 '5 inches in length, the extremes among the small 

 individuals being 64 and 67 inches, those amongst the medium 67 

 and 70 inches, and those among the large 70 and 73 inches. 

 Then suppose that by the principle of reproductive divergence the 

 individuals were separated into two groups "of an average of 64 and 

 73 inches in length. Then it follows that these groups would 

 (approximately) contain individuals varying between 6 2 '5 and 65*5 

 inches, and 71'5 and 74*5 inches respectively. That is to say, 

 considerably smaller and considerably larger individuals would be 

 formed than were originally present. Also if the principle of 

 reproductive divergence continues to act amongst the two varieties 

 of the original species formed, the individuals will continue diverg- 

 ing more and more in respect of this characteristic, with which the 

 reproductive power of the organism is correlated. Also, if it be 

 granted, that on an average, the more widely any two individuals 

 differ in size, the greater is the relative degree of sterility between 

 them, it follows that in course of time the individuals of the two 

 varieties will become mutually sterile : or in other words, that from 

 the original species two new species will have arisen. 



Having demonstrated the correctness of the second part of the 



