90 OKIGIN OF LIFE IN AMERICA 



less than a hundred and twenty-two species found in temperate 

 and northern Europe, and also in North America, reasoning 

 that since the greater number of them also live in northern 

 Asia they must have spread from one continent to another 

 across Bering Strait. He does not definitely state that he 

 considers the Strait to have been a land surface at the time 

 these species are supposed to have passed across it. Still he 

 emphasises the intimate relationship of the Alaskan and the 

 opposite Siberian shore fauna, by citing the names of four 

 species of Hemiptera which are confined to these two 

 countries. 



A rather striking example clearly illustrating the gradual 

 advance of a genus across Asia, and its final immigration 

 into North America, is shown by Parnassius, a genus of 

 butterflies to which the European " Apollo " belongs. The 

 Apollo group of butterflies originated, according to Mr. 

 Austaut, towards the latter part of the Tertiary Era in Asia, 

 at the time when some of the great mountain chains of that 

 continent were being formed. I alluded to the European 

 species of Apollo in my work on European Animals, gave a 

 map of their distribution and enlarged upon their origin. I am 

 in full agreement with Mr. Austaut's views as to the Asiatic 

 centre of dispersal. The only point in which I differ is with 

 regard to the period of its origin, which I believe to have 

 been considerably earlier than Mr. Austaut* thinks likely. 



In Europe we possess only three species of Parnassius, 

 whereas there are thirty-five in Asia, Turkestan being the 

 richest country in Apollos. Some species are peculiar to the 

 Altai Mountains in Siberia, and as we go eastward new forms 

 replace those with which we had become familiar. In Amur- 

 land there are others, and yet when we cross Bering Strait to 

 Alaska we still meet with an Apollo (P. nomion) which also 

 occurs in the Altai Mountains, while the Californian Apollo 

 (P. clodius) is very closely related to Parnassius clarius of 

 these same mountains. The Asiatic character has thus been 

 retained by two of the American species. Two others, Par- 

 nassius thor of Alaska and Parnassius sminthus, are quite 

 distinct from any others, indicating that the passage from 



* Austaut, J. L., "Les Parnassians," pp. 6263. 



