32 MEMOIRS OF TEE NATIONAL ACADEIMY OF SCIENCES. 



•and specialization of larval forms in the Boinbyces is due to changes in their environment after 

 they had effected their descent from their Lithosian ancestry. It was from adaptation to totally 

 new surroundinos which at once broke up the old simplicity of shai)C of their early ancestry and 

 induced a striking i)lasticity of form and of structural features. 



Such changes as these could not have been brought about so recently as the Quaternary 

 period, but must have been most active during the late Mesozoic and tluonghout the Tertiary. 

 Probably the date of the appearance of the Bombycine phylum was coeval with the appearanweof 

 the Cretaceous forests. 



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

 great many forms 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 gaps in their numbers— few links 

 which are missing. 



We would suggest, then, that the plasticity of the larval forms of the Bombyces, 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 au evidence that they have had a 

 high antiquity. When, for example, a group like the Heterocampina> is entirely wanting in Europe 

 and the western portion of Xorth 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 America. Lingula was once abundant all over the globe; it now only 

 lives along portions of the American and Asiatic and Australian coasts. Linmlus 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 Peninsnla to Japan, having 

 become extinct in other parts of the world. 



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

 geological extinction, and also to the great plasticity or marked difference in the larvje, as compared 

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

 environment which took place during the later Mesozoic and Tertiary periods, and which reacted 

 ■on the insects in tlieir early rather than later stages. 



This incongruity between the larval and adult stages, then, was probably 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 Xoctuidse 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 Europe, geologically speaking, although 

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

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



Were fossil Bombyces ever to be found in Europe, we should expect to discover among them 

 representatives of the Cochliopodid^, of the Attacine Saturniida', Ceratocampida', and Notodou- 

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

 of Africa. 



Among the Notodontida^ the Heterocampidiie, for example, now confiued to eastern North 

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

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

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

 it may have existed in Europe, from which the Euroi)ean Cerurhiw may have evolved, unless the 

 type migrated from Asia. There Is a species of Stauropus in India, though there are few Noto- 

 dontians in that country, and southeastern Asia is evidently the center of development of the bulk 

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

 €xtensively in Europe, perhajis as the result of the cold of the Glacial epoch. 



