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 (z.¢., J and m breeding with 
sand S or/ and L) give birth to, on an average, only 95 offspring, 
and 100 individuals breeding with considerably smaller or larger 
individuals (7.e., S or LZ 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 MI, 120 L1. 
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 Mand 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 JZ 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 62°5 and 65°d 
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 
