228 APPENDIX II — INTENSIVE SEGREGATION. 



for the mogt part distinct, reinforced by partial segregate fecundity 

 which may or may not be accompanied by shghtly divergent sexual 

 instincts. There is also some isolation resulting from the fact that the 

 plants on which B. arthemis seeks to deposit its eggs are chiefly the 

 birches and willows of the hilly country, while B. astyanax prefers fruit 

 trees of the Rosacese family and other plants that are found in the 

 more open country. These are, as I have shown in my paper on 

 "Divergent Evolution through Cumulative Segregation," exactly the 

 conditions that produce, in successive generations, increasing degrees 

 of segregate fecundity. 



(4) Cumulative Segregation in the Formation of the above Species. — I 

 judge that in the relations to each other of these three species we have 

 the results of divergent evolution through cumulative segregation 

 very clearly illustrated. In the earlier stages of divergence in this 

 genus, Basilarchiaarchippus, with its fondness for the open fields, must 

 have become partially separated from the parent form from which both 

 B. astyanax and B. arthemis have since sprung. The separation may 

 have been in some measure due to the methods of escaping from 

 enemies ; for we find that the form that has kept to the open country 

 has through protective selection gained a very close resemblance to the 

 coloring of Anosia plexippus, which is protected by its disagreeable 

 qualities. The other form has probably gained compensative advan- 

 tages by keeping closer to the woodlands. But the partial segrega- 

 tion thus produced would never have resulted in constant specific dif- 

 ferences if segregate fecundity had not arisen between the two forms. 

 We may believe that some form of impregnational segregation (either 

 segregate structure, segregate fecundity, or segregate vigor) was early 

 introduced, and that under the protection of this barrier the specific 

 distinctions of the two forms became fully established, though even 

 now the barrier is not so complete as to entirely preclude hybrids 

 between B. archippus and each of the other species. Examples of 

 both these hybrids are described by Scudder. 



While this segregation was being completed, one of the two forms 

 thus created must have become subject to a new set of segregative in- 

 fluences arising from wider distribution with diversity of climate and 

 of habits of feeding, reinforced by a slight degree of segregate fecun- 

 dity. B. astyanax and B. arthemis are the two species resulting from 

 this last segregation, and the process is so far from being complete 

 that wherever the areas of these two species overlap a hybrid form, 

 which has been known as B. proserpina, appears. That it is a hybrid 

 is proved by the fact that it "varies most toward astyanax where this 

 prevails, and most towards arthemis where that prevails;" that it is 



