98 Heredity and Variation in Modern Lights 



fertility once re-established, the sterility does not return in the later 

 progeny, a fact strongly suggestive of segregation. Now if the sterility 

 of the cross-bred be really the consequence of the meeting of two 

 complementary factors, we see that the phenomenon could only be 

 produced among the divergent offspring of one species by the acquisi- 

 tion of at least two new factors ; for if the acquisition of a single 

 factor caused sterility the line would then end. Moreover each factor 

 must be separately acquired by distinct individuals, for if both were 

 present together, the possessors would by hypothesis be sterile. And 

 in order to imitate the case of species each of these factors must be 

 acquired by distinct breeds. The factors need not, and probably would 

 not, produce any other perceptible effects ; they might, like the colour- 

 factors present in white flowers, make no difference in the form or 

 other characters. Not till the cross was actually made between the 

 two complementary individuals would either factor come into play, 

 and the effects even then might be unobserved until an attempt was 

 made to breed from the cross-bred. 



Next, if the factors responsible for sterility were acquired, they 

 would in all probability be peculiar to certain individuals and would 

 not readily be distributed to the whole breed. Any member of the 

 breed also into which both the factors were introduced would drop 

 out of the pedigree by virtue of its sterility. Hence the evidence 

 that the various domesticated breeds say of dogs or fowls can when 

 mated together produce fertile offspring, is beside the mark. The 

 real question is, Do they ever produce sterile offspring ? I think the 

 evidence is clearly that sometimes they do, oftener perhaps than is 

 commonly supposed. These suggestions are quite amenable to ex- 

 perimental tests. The most obvious way to begin is to get a pair of 

 parents which are known to have had any sterile offspring, and to 

 find the proportions in which these steriles were produced. If, as I 

 anticipate, these proportions are found to be definite, the rest is 

 simple. 



In passing, certain other considerations may be referred to. First, 

 that there are observations favouring the view that the production of 

 totally sterile cross-breds is seldom a universal property of two species, 

 and that it may be a matter of individuals, which is just what on the 

 view here proposed would be expected. Moreover, as we all know 

 now, though incompatibility may be dependent to some extent on 

 the degree to which the species are dissimilar, no such principle can 

 be demonstrated to determine sterility or fertility in general. For 

 example, though all our Finches can breed together, the hybrids are 

 all sterile. Of Ducks some species can breed together without pro- 

 ducing the slightest sterility ; others have totally sterile offspring, and 

 so on. The hybrids between several genera of Orchids are perfectly 



