CLIMATIC VARIETIES 179 



clear why their interbreeding does not produce actual bryoniae 

 occasionally. If this occurred, the probability is that the fact 

 would be known to collectors, at least in the British localities. 

 The absence of true bryoniae must, I think, be taken to mean 

 that some essential factor is absent from these intermediates. 



To sum up the evidence, the facts that are clear may be thus 

 enumerated : 



1. Napi and bryoniae, or in the Italian Alps, napaeae and 

 bryoniae frequently meet each other. 



2. They cross without difficulty, producing fertile offspring. 



3. But in the levels at which they overlap there is no inter- 

 mediate population, and only occasional intermediate individuals. 



4. In certain parts of the distribution of napi similar inter- 

 mediates sometimes occur, and at one place (Modling) they are 

 so frequent as apparently to constitute a colony. 



5. As to the genetic relations of the two forms there is no 

 complete certainty. Indications of segregation have been ob- 

 served in some cases, but there are several factors concerned and 

 they are liable to some disintegration. 



Another form in which I tried to investigate the same problem 

 is Coenonympha arcania, which has one Alpine form known as 

 darwiniana, and another, satyrion. In calling satyrion a form of 

 arcania I follow Staudinger and other authorities, but I have 

 never been quite satisfied that it should be so regarded. The 

 differences between arcania and darwiniana are essentially dif- 

 ferences of degree; C. arcania occurs in places where there is 

 cover, and reaches up the valleys usually as high as the mixed 

 woods of deciduous trees, which is about 2,500 feet. The variety 

 darwiniana, on the contrary, is an insect of treeless hillsides, and 

 I regard it as a dwarf and possibly a stunted form. It would not 

 greatly surprise me to find that with the application of good 

 conditions arcania could be raised from darwiniana eggs, or that 

 if arcania larvae were starved they might give rise to darwiniana 

 butterflies. I have been unsuccessful in trying to rear the species, 

 having lost the larvae by disease. Usually one does not catch 

 arcania and darwiniana on the same ground, and as Festuca ovina 

 — a typically hill-side grass — is a common food-plant of dar- 



