320 NATURAL SCIENCE [November 
of specific distinctness which the principle of Reproductive Divergence 
is propounded to explain, we will accept, for the sake of argument, 
that parents of the same size breed true. Then of the 120 Ss of the 
first generation 24 will copulate with small females and 24 each 
with the other four sets; hence there will be produced only 29 Ss. 
These 29 will have to copulate with nine sets, and so on. The same 
applies to Ll. Or to put it generally, if A is the number of indi- 
viduals of each original set, a the number of original sets, x the sur- 
plus fertility, n the number of generations, then under the proposi- 
tions adduced by Mr Vernon, 
nti A 
os =i ae y ((a-1)'+1).((@—1°+1)... (a= +1) 
In our case the numbers of S in the succeeding generations will, 
therefore; be—l= 1:20; TL =29 e431 = Ore 
That is to say, after the fourth generation, the largest and 
smallest specimens will be weeded out, and this result will not 
materially be altered, even if we assume that the largest and 
smallest individuals are mutually absolutely sterile. (Compare also 
Galton’s regression towards the mean.) 
Although Reproductive Divergence does not achieve what Mr 
Vernon claims for it, it is not altogether to be rejected under other 
premises than those accepted by Mr Vernon. There are certain 
species, for instance among Lepidoptera, which vary in the same 
locality in such a way, that there are two well-marked varieties 
which breed freely with one another, but produce comparatively few 
intergraduate specimens, the offspring belonging mostly to the one 
or to the other variety.1 Here Reproductive Divergence may 
eventually have free play, and then necessarily will evolve incipient 
dimorphism into complete dimorphism, and in so far Reproductive 
Divergence might be called a factor in evolution. 
KarL JORDAN. 
ZOoLoGicAL Musrum, TRING. 
1 Standfuss, ‘‘ Handbuch f. Schmetterlingssammler,” 1895.—See also Giard, Natural 
Science I. p. 388 (1892) ; Romanes, ibid. p. 398. 
