32 MEMOIRS OF THE NATIONAL ACAT)E:\IY OF SCIENCES. 



anil specialization of larval forms in the Bombycos is duo to changes in their einiroiinient after 

 they had ett'eeted their descent from their Lithosian ancestry. It was from adaptation to totally 

 new surroundings which at t)nce broke up the old simplicity of shape of their early ancestry and 

 induced a striking plasticity of form and of structural features. 



Such changes as these could not have been brought about so lecently as the Quateinary 

 peiiod, but must have been most active during the late Mesozoic and throughout the Tertiary. 

 Probably the date of the appearance of the liSombycinc jihylnm was coeval with tlie appearanaeof 

 the Cretaceous forests. 



We have always maintained that the Bombyces are a very old type, which have lost a 

 great many tbrms by geological extinction. In number of species the type is at present far less 

 numerous than the Noctuina. The ranks of the latter have not been thinned by the ravages of 

 geological time; on the contrary, there are few and unimportant gai)s in their numbers— few links 

 which are missing. 



We would suggest, then, that the plasticity of the larval forms of the Bond)yces, especially 

 in the "lower," or to speak more correctly, the more primitive and in a degree generalized, families, 

 is due to the great changes in their environment during the Cretaceous and Tertiary periods. 



This is indicated by the facts in geographical distribution to be stated in more detail further 

 on. When species are widely distributed this is to be taken as an evidence that they have had a 

 high antiijuity. When, for example, a group like the TIeterocanii)ina' is entirely wanting in Europe 

 and the western portion of North America, such great gaps in distribution are naturally to be 

 attributed to geological extinction. 



It will be recalled that the opossum and other marsupials are extinct in Europe, though existing 

 at present in Australia and .Vmerica. Lingula was once abundant all over the globe; it now only 

 lives along portions of the iVmerica n and Asiatic and Australian coasts. Limulus was represented 

 by several species in the Jurassic of Europe but now only occurs on the northeastern shores of 

 North America and the eastern shores of Asia from the Malaysian Peninsula to Jai)an, having 

 become extinct in other parts of the world. 



In like manner the great gaps in the genera of onr existing Bombyces are probably due to 

 geological extinction, anil also to the great plasticity or marked difference in the larva', as compared 

 with the homogeneousuess of the imagines, these being due to the widespread changes in the 

 envirf)iiment which took place during the later Mesozoic and Tertiary i)eriods, and which reacted 

 on the insects in their early rather than later stages. 



This incongruity between the larval and adult stages, then, was jjrobably most marked in the 

 periods before the (Quaternary, while since then there has been divergence. We have some reason 

 to suppose that the families of Noctnida^ and Geometrida-, so numerous in species, were largely 

 evolved during the Pliocene and (Quaternary. 



Where a family or subfamily is equably developed both in the Old and New worlds, we are 

 inclined to suppose that it was a recently evolved group. 



It is well known that America has lagged behind I'lurope, geologically siu'aking, although 

 America is the older continent as such; the process first of specialization anil then of extinction 

 has gone on more rapidly in the Old World, or at least the westcm jjortion of it. 



Were fossil Bombyces ever to bo found iu Europe, we should expect to discover among them 

 representatives of the Cochliopodida', of the Attacine Saturniida', Ceratocam])ida', and Notodon- 

 tidie, now characteristic of North and South America or of the tropical regions of Asia and perhaps 

 of Africa. 



Among the Notodontida' the Ileterocampida^, for example, now contined to eastern North 

 America, Central America, and western South America, may have flourished in Europe contem- 

 poraneously with the sequoia, magnolia, liquidambar, gum tree, and other existing types of 

 vegetation now extinct in Europe. Although Macrurocampa is an American genus, some form like 

 it may have existed in Europe, from which the Eurojjean Ccrur'nuv nuiy have evolved, unless the 

 type nugrated from Asia. There is a species of Stauropus iu India, though there are few Noto- 

 dontiaus iu that country, and southeastern Asia is evidently the center of development of the bulk 

 of the European genera of Bombyces, geological extinction in these moths having gone on very 

 extensively in Europe, perhajjs as the result of the cold of the Glacial epoch. 



