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 [From the Proceedings of the American Association for the Advancement of Science, 
 Detroit Meeting, Aujuet, 1875.] ,ji 
 
 The Effect of the Glacial Epoch upon the Distribution of 
 Iksects in North America. By Aug^R^^Gkote, of Buffalo, 
 
 N. Y. 
 
 
 From the condition of an hypothesis, the Glacial epoch has 
 been elevated into that of a theory, by the explanations it has 
 afforded to a certain class of geological pheromena. The present 
 paper endeavors to show that certain zoological facts .are consist- 
 ent with the presence in past times of a vast progressive field of 
 ice gradually extending over large portions of the North American 
 Continent and moving from the north to the south. These facts 
 are in the present instance afforded by a study of the Lepidoptera, 
 certain kinds of butterflies and moths now inhabiting the United 
 States and adjacent territories. Before proceeding with the 
 subject, a brief statement of some of the phenomena assumed to 
 have attended the advent of the Glacial Epoch is necessary. 
 
 At the close of the Tertiary, the temperature of the earth's 
 surface underwent a gradual change by a continuous loss of heat. 
 The winters gradually became longer, the summers shorter. The 
 tops of granitic mountains in the east and west of the North 
 American continent, now in summer time bare of snow and 
 harboring a scanty flora and fauna, became, summer and winter, 
 covered with congealed deposits. In time the mountain snows 
 consolidated into glacial ice which flowed down the ravines into 
 
 (222) 
 
 S-j 
 
I 
 
 fc^^,-'<^^ 
 
 223 
 
 B. NATURAL HISTORY. 
 
 the valleys. Meanwhile the northern rejjions of the continent, 
 which may have iiiaiignrated, siihniittcd cxteiidcdly to the same 
 plicnomena. Glacial icu, llrst made on elevations, linally formed 
 at, an<l ponred over lower levels. Glacial streams linally nnited 
 to form an icy sea whose frozen waters slowl}' plowed the surface 
 of the ro( ks ; and whose waves, in their movement from north to 
 south, absorbed the local glaciers in their course, and extended 
 over all physical barriers into the .Southern States and down the 
 Valley of tlie Mississippi. To the main Ice-sheet, the Appalach- 
 ians and Koclv}' Mountains are supposed to have contributed their 
 local glaciers. Before this frozen deluge the animals must have 
 always rctieated. The existing insects of the pliocene must, in 
 submitting to the change of climate which accompanied the ad- 
 vance of the glacier, ha\e quitted their haunts with reluctance, 
 and undergone a severe struggle for existence, no matter how 
 gradually the}' had been prepared for the encounter. We must 
 expect that nmltitudes of specific forms ultimately perished of 
 whose remains no traces have been preserved. 
 
 Such being a brief statement of the outlines of the opening of 
 the Glacial P2poch, we turn to some facts offered by a study of 
 our existing species of butterflies and moths. Tlie tops of the' 
 White Mountains and the ranges of mountain elevations in 
 Colorado, offer us particular kinds of these insects living in an 
 isolated manner at the present day and confined to their respec- 
 tive localities. In order to find insects like them we have to ex- 
 plore the plains of Labrador and the northern portion of the 
 North American Continent, in regions offering analogous condi- 
 tions to those obtaining on the summits of these mountains. The 
 genera Oeneis and Brenthis among the butterflies, and Anarta 
 and Agrotis among the moths, are represented by the same or 
 similar species in all of the above mentioned localities. In the 
 case of the White Mountain Butterfly, Oeneis semidea, we have a 
 form sustaining itself on a very limited alpine area on the top of 
 Mount Washington. • Although there is some doubt that precisely 
 
 » Sec Mr. Si'iuMor's nrticlo in tlie •' ecology of New Ilampsliire," 1 ,342. Mr. .SciuWer 
 first pointed ont the existence of alpine and subalpine fa\inal belts on Monnt Wa.sliing- 
 ton. and iutere.sUnjrly remarks " tliat if the .suniniit of Jit. Wasliington were somewhat 
 less than two thousand feet higher it would reach the limit of perpetual snow." Con- 
 eult also, an earlier paper of great value liy Dr. A. S. Packard, Jr., on "The Insect 
 Fauna of the Summit of Mount \Va(:hli)gton as compared wit! that of Labrador" (these 
 rrncecdings. Vol. XVI, 1.51). l>r. "ackard. In ccmiparing tlio climate of the two locali- 
 ties. Bays : "The seasons correspon.l very exactly, as the snow melts in the early sum- 
 mer, uud ice Ib formed early iu tlie autumu ut about the same dates." 
 
 .-> 
 
 \ 
 
 I 
 
 .G(^ 
 
 n 
 
VV 
 
 .J 
 
 B. NATURAL IIISTOUr. 
 
 224 
 
 the same form has been discovcri'd in Colorailo, the fact remains 
 tliat huttertlies exceedingly lilu" it, tlioiigii rooistored by us under 
 dill'ercnt specilic names, live in Lal)rador and Colorado. 
 
 ■Whether the White ^lountain Butterlly be, as suspected by 
 Lederer, a local modilication of some one of the Labradoriau 
 forms or not, the geoj5rai)liical distribution which its genus enjoys 
 cannot be meaiungless. The question comes up, with regard to 
 the White Mountain Butterlly, as to the manner in which this 
 species of Of'/<e/.s attained its present restricted geographical area. 
 How did the White INIouutain Butterlly get up the White 
 Mountains ? And it is this question that I am disposed to answer 
 by the action attendant on the decline of the Glacial Epoch. 
 
 I have before brielly outlined the phenomena attendant on the 
 advance of the Ice-sheet, anel I now dwell for a uuunent on those 
 ^vhichmust equally be presumed to have accompanied its retire- 
 ment. I>Iany of the features of its advance were repeated m 
 reverse order on the subsidence of the nuiin Iccsheet or Glacial 
 sea. The local glaciers appeared again separate from the main 
 body and filled the valleys and mountains and ravines, running 
 thus at variance with the main body of the C; lacier, being deter- 
 mined by local topography. A reversal of the temperature 
 shortened the winters and Uuigtheued the summers. Ice-loving 
 kinds of insects, such as our White Mountain Butterfly, hung on 
 the outskirts of the main Ice-sheet, where they found their fitting 
 conditions of temperature and food. The main Ice-sheet had 
 pushed them insensibly before it, and, during the continuance of 
 the Glacial Epoch, the geographical distribution of the genus 
 Oeneis had been changed from a high northern region to one which 
 may well have included portions of the Southern States. And, 
 on its decline, the Ice-sheet drew them back again after itself by 
 easy stages; yet not all of them. Some of these butterflies 
 strayed by the way, delayed by the physical nature of the country 
 and destined to plant colonies forever separate from their 
 companions. Wlien the main Ice-sheet left the foot of the White 
 Mountains, on its long march back to the pole where it now seems 
 to rest, some of these wayward, flitting, Oeneis butterflies were 
 left behind. These had strayed up behind the local glaciers on 
 Mount Washington, and so became separate from the main body 
 of their companions which journeyed northward, following the 
 retirement of the main Ice-sheet. They found in elevation their 
 
 i 
 
 r 
 
 •jiil'irflT" 
 
-v--** '■* 
 
 »Wi f i».T — W 
 
 / 
 
 225 
 
 B. NATURAL HISTORY. 
 
 congenial food and climate, and tliey have followed these gradu- 
 ally to the top of the mountain, which tlicy have now attained and 
 from which tiiey cannot now retreat. Far olf in Labrador, the 
 descendants of their ancestral companions t\y over wide stretches 
 of country, while they appear to be m prison on the top of a 
 mountain. 
 
 I conceive that in ihis way the mountains generally may have 
 secured their Alpine animals. Tlie Cilacial Epoch cannot be said 
 to have expired. It exists even now for high levels above tha 
 sea while the Laplander and Escpiimaux find it yet enduring in the 
 far North. Our yearly winters are fractions of the (ihiciai year. 
 Had other conditions been favorable, we might now fnid Arctic 
 man living on snow-capped mountains in tlie Temperate zones. 
 
 At a height of between 5,000 and 0,200 feet above the sea and 
 at a mean temperature of about forty-eight degrees during a short 
 summer, the White Mountain Butterflies {Oeneis semidea), yet 
 enjoy a climate like that of Labrador witiiin the geographical 
 limits of New Hampshire. And' in tlie cases of the moths- an 
 analogous state of things exists. Tlie species Anarta melanopa is 
 foimd on Mount Washington, the Rocky Mountains and Labrador. 
 Agrotis islandica is found in Iceland, Labrador, the White Mouti- 
 tains, and, perhaps, Colorado.'' As on islands in the air, these 
 insects have been left by the retiring of the ice-tlood during the 
 opening of the Quaternary. 
 
 On inferior elevations, as on Mount Katahdin in Maine, where 
 we now find no Oeneis butterflies, these may liave formerly ex- 
 isted, succumbing at last to a climate gradually increasing in 
 warmth from which they had no escape ; ivhile the original coloni- 
 zation in the several instances must have always greatly depended 
 upon local topography. 
 
 In conclusion, I have briefly endeavored to show that the 
 present distribution of certain North American insects may have 
 been brought about by the phenomena attendant on the Glacial 
 Epoch. The discussion of matters connected with this theoretic 
 period of the earth's history, still, as it now ap)iears, brings out 
 more and more a clearer conception of its actuality. I hope that 
 
 «I have Bince (Psyche, 1, 131) recorded the first indication of the occurrence of the 
 Arctic Larta liossii on Slount Washington, from a single specimen taken by Mr. B. 
 Ficlsman Mann, above tlie tree line. 
 
 »I believe Dr. Packard's id'intiflcation of this species, in Prof. Hayden's Reports, 
 is Incorrect. The Coloradian species is Agrotis auxiliarU, Grote. 
 
 A. A. A. 8. VOL. XXIV. B. (15) 
 
 ,j*i 
 % 
 
B. NATnnAt IIISTOKT. 
 
 22G 
 
 //- 
 
 my present statements may draw the attention of our zoologists 
 more fully to the matter, seeing tiiat we have in our own country 
 fields for its full exploration. And I permit myself to believe, that 
 testimony as to the former existence of a long and widely spread 
 winter of the years, is offered in evidence through the frail, l)rown, 
 Oeneis butterflies, that live on the tops of the mountains. 
 
 [Printed at the Sai.em Puess, June, 187U.]