ISOLATION 291 



suits when interbred. 5 Here is a typical case cited by 

 Kellogg. "We know of numerous species of butter- 

 flies which appear in different seasons of the year in 

 different colour-pattern. This is not a colour change 

 in individuals but results from an earlier or later 

 hatching of eggs laid in the autumn or summer be- 

 fore. These eggs may, indeed, be all of one batch or 

 lot, laid by a single female. Some of these eggs 

 hatch in the spring; the butterflies that come from 

 these spring larva? are of one colour-pattern; some of 

 the eggs, however, delay hatching until summer ; from 

 these larva? come butterflies of another colour-pattern ; 

 some of the eggs even go over until fall before hatch- 

 ing; these latest butterfly individuals may be of a 

 third colour-pattern. The colour-pattern here must 

 have some fixed relation to the time or season of hatch- 

 ing the eggs ; it is not a result of isolation. But the 

 condition well illustrates the actual existence of a bio- 

 logical isolation within a species: the spring butter- 

 flies must mate among themselves, the summer indi- 

 viduals among themselves, and the fall butterflies 

 among themselves. Within the one species are three 

 biologically isolated groups of individuals restrained 

 from interbreeding." 8 



It may also happen in the plant world (this example 

 is quoted by Romanes from Darwin) , that pollen from 



s Darwin and After Darwin, Vol. III. 

 8 Darwinism, To-day, pp. 243-244. 



