364 REV. J. T, GIJLICK OK 



in whidi their areas overlap, the intergrade forms are compara- 

 tively abundant. Moreover, the difference in the male genitalia 

 of B. Astyanax and B. Arthemis, though much less than that 

 which appears when either of tliese is compared with B. Ar- 

 chippus, is such as indicates a considerable degre§ of infertility. 



In these two species we have then a good example of partial 

 Segregation through distribution over areas, which, thougli over- 

 lapping, are for the most part distinct, reinforced by partial 

 Segregate Eecundity which may or may not be accompanied by 

 slightly divergent sexual instincts. There is also some Segrega- 

 tion resulting from the fact that the plants on which 5. Artliemis 

 seeks to deposit its eggs are chiefly the birches and willows 

 of the hilly country, while B. Astyanax prefers fruit-trees of the 

 Eosaceae 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 

 tlie conditions that produce, in successive generations, increasing 

 degrees of Segregate Fecundity. 



Oumulative Segregation in tJie Formation of the atove 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 Arcliippiis with its fondness 

 for the open fields must have become partially separated from the 

 parent form from which both B. Astyanax and B. Artliemis have 

 since sprung. The separation may have been in some measure due 

 to what I have called Protectional Segregation ; for we find that 

 the form that has kept to the open country lias through protective 

 selection gained a very close resemblance to the colouring of 

 Anosia plexippus, which is protected by its disagreeable qualities. 

 The other form has probably gained compensative advantages by 

 keeping closer to the woodlands. But the partial Segregation thus 

 produced would never have resulted in constant specific diff'erences 

 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 

 Yigour) 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 



