228 APPENDIX II—INTENSIVE SEGREGATION. 
for the most part distinct, reinforced by partial segregate fecundity 
which may or may not be accompanied by slightly divergent sexual 
instincts. Thereisalsosome 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 Rosaceze 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, Basilarchia archippus, 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 plextppus, 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.astyanaxand 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 
