244 DARWINISM TO-DAY. 



change in individuals but results from an earlier or later 

 hatching of eggs laid in the autumn or summer before. 

 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 larvae are of one col- 

 our-pattern ; some of the eggs, however, delay hatching until 

 summer; from these larvae come butterflies of another 

 colour-pattern ; some of the eggs even go over until fall 

 before hatching ; 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 hatching of the 

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

 illustrates the actual existence of a biological isolation within 

 a species : the spring butterflies must mate among them- 

 selves, the summer individuals among themselves, and the 

 fall butterflies among themselves. Within the one species 

 are three biologically isolated groups of individuals re- 

 strained from inter-breeding. Suppose the individuals of 

 a bird species show among themselves a tendency to vary 

 in their breeding time ; some are ready to breed early, others 

 delay mating. Roughly segregated into two groups, early 

 breeders and late, the individuals of the two groups would 

 obviously tend to breed each inside its own group. Hut- 

 ton 13 actually records the occurrence of two varieties of the 

 shore-bird, CEstrelata neglecta, in the Kermadec Islands, 

 which live together but breed at different times. A pelagic 

 crustacean living near the shore increases rapidly in num- 

 bers ; some individuals find themselves able to live on the 

 shore in pools between tide-lines. The pool dwellers breed 

 together: the pelagic individuals breed together; a biologic 

 isolation in truth an isolation partly topographic might 

 soon come to exist. Any variation in habits of life among 

 individuals living in the same locality, which tends to deter- 

 mine that breeding shall be roughly restricted within cer- 

 tain groups produces biologic isolation ; such variation might 



