154 Heredity. 



fore bo more likely than a part which has not recently 

 varied to vary still farther. 



Walsh has called attention ("Proc. Entomolog. Soc.," 

 Philadelphia, October, 1863, p. 213) to what he calls 

 the "Law of Equable Variation," which is, "if any 

 given character is very variable in one species of a group, 

 it will tend to be variable in allied species, and if any 

 given character is perfectly constant in one species of a 

 group, it will tend to be constant in allied species." 



This is by no means an absolute law, but simply a gen- 

 eral rule. Darwin points out that something of the same 

 kind occurs in domesticated races, and that in the forms 

 which are now undergoing rapid improvement those 

 parts or characters which are most valued vary the 

 most. 



We can readily see that circumstances which cause a 

 certain part to throw off gemmules, and thus induce 

 variability, in one species, will be likely to produce the 

 same effect on allied species living under similar circum- 

 stances. We can also understand that the divergent 

 modification which has resulted in the formation of 

 several species or races from a parent form, will in it- 

 self be a cause of still further modification in the same 

 general direction. 



Another well-known law, of which many examples will 

 be given in Chapter IX. is that secondary sexual char- 

 acters are highly variable. In the chapter on this sub- 

 ject I shall show that the distinctive sexual characters 

 of a species are usually due to recent modification. 

 Their great variability is therefore due to the same 

 cause as that which renders specific characters more 

 variable than generic, and is exactly what our theory 

 would lead us to expect. 



