Butterflies of the Mountain Summits. 



87 



species (showing only certain slight variations which have 

 given them separate sub-species ranking) , present to us one of 

 the most important and interesting special biological problems 

 in butterfly life. Nor is it a problem limited to butterfly dis- 

 tribution, but it is one that arises in the consideration of the 

 distribution of any other mountain-top insects or other animals. 



The problem has had much attention, and its solution seems 

 to be that most of these mountain peak species are the persist- 

 ing representatives of almost unmodified descendants of Glacial 

 Epoch forms, the ofifspring of stranded individuals left on the 

 summits at the time of the retreat of the great ice-fields. The 

 glacial species extended across the continent in glacial times. 

 With the retreat northward of the ice-sheets some animal and 

 plant kinds retreated with it, but others followed the withdraw- 

 ing local glaciers up the mountain canons. The ones that went 

 north are to-day Arctic species ranging across the northern 

 part of the continent. The ones that went up the mountains are 

 to-day alpine species existing in little isolated groups on widely 

 separated mountain summits in mid-continent latitudes. Some 

 alpine forms extend north along the summit of a mountain 

 range, as the Rockies or the Sierra Nevada, until they reach 

 Arctic or sub-Arctic conditions and then range across the con- 

 tinent. Altitude equals latitude as regards biological environ- 

 ment, and our little butterflies of the mountain tops in sunny 

 California are really living in and enjoying the Arctic condi- 

 tions of their glacial time ancestors, and of their far northern 

 cousins of to-day. 



There is interesting experimental proof of the fact that the 

 Arctic and alpine forms are the older, and the southern and low- 

 land forms the newer, in a species of butterfly represented in 

 Europe by several different varieties. This proof is that 

 afforded by the work of Fischer, a former student of the Uni- 

 versity of Zurich. The butterfly in question is the widespread 

 and familiar mourning-cloak, or Camberwell beauty, Euva- 

 nessa mtiofa. It occurs almost everywhere in the north tem- 

 perate zone, but while it shows little variation in North 

 America, in Europe it exists in a series of distinct, although 

 gradating varieties, each characteristic of a rather narrow 

 latitudinal zone. 



