320 T. D. A. COCKERELL. 



not migration take place into the neotropical region. That it did 

 not take place, at all events beyond the isthmus, is evidenced by the 

 facts above quoted from Mr. Whymper's "Appendix;" and the 

 reason of this no doubt is, that the isthmus itself was submerged, 

 and all connection between North and South America cut off. This 

 question of the submergence of the Isthmus of Panama has been 

 fully discussed by various naturalists, and need not be enlarged upon 

 here. 



It is impossible in the present paper to give more than this bare 

 outline of the subject, but I believe the conclusion is justified, that 

 the central region fauna was practically stamped out during the 

 glacial epoch ; and that the present fauna is derived from the boreal 

 faun?e which survived to the east and to the west, and the southern 

 fauna which survived in Mexico. This view seems to be supported 

 by a consideration of the present distribution of species, as well as 

 by geological evidence. 



REMNANTS OF THE ANCIENT FAUNA. 

 Mr. Scudder's tertiary insects of course date a long way back, and 

 we have no knowledge of the entomology of Colorado immediately 

 before the glacial epoch. But whatever it may have been like, it 

 is reasonable to suppose that there would be some few survivors 

 left, if we only knew which they were. I believe Anthracopteryx 

 hiemalis is such a survivor ; it is a peculiar species of wide affinities, 

 the only representative of its genus ; and moreover, it is not afraid 

 of cold, appearing very early in the year. Among Lepidoptera, it 

 may be that Neophasia menapia is a survivor, and also some of the 

 high-alpine species. But this is mere speculation — it is enough to 

 suggest the probability of such a thing. 



POST-GLACIAL DEVELOPMENTS. 



Excepting the remnants of the ancient fauna, all the strictly en- 

 demic element in the Rocky Mountains is o? post- glacial origin — 

 that is, according to the views here set forth. This means a good 

 deal, if it is actually the case, as I believe. Under certain circum- 

 stances, species develop quickly, and we have, at least among insects 

 and flowering plants, a great array of new species coming into ex- 

 istence. Such species are closely allied to species from which they 

 sprang, and to each other, so as to give rise to much dispute as to 

 their validity — as an example, one may cite the genus Argynnis, 

 which has been very productive of post-glacial species in America. 



