32 
MEMOIRS OF TEE XATIOKAL ACADEIMY OF SCIENCES. 
specialization of larval foriiis in the Bombj^ces is due to changes iii tlieir environment after 
they had effected tlieir descent from their Lithosian ancestry. It was from adaptation to totally 
new surroundings which at once broke up the old simidicity of shape of their early ancestry and 
induced a striking iilasticity 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 throughout the Tertiary. 
Probably the date of the appearance of the Bombycine i^hyluni was coeval with the appearaii&*e of 
the Cretaceous forests. 
Wo liave always maintained that the Bombyces are a very old type, wliich 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 unimiKutant 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 ^nower,” or to speak more correctly, tlie more xirimitive 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 bo stated in more detail further 
on. When species are widely distributed this is to be taken as an e\ideuce that they have had a 
Mgh antiquity. When, for example, a group like the Heterocampiiia? 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 bo recalled that the opossum and other marsuj)ials ai^e extinct in Europe, though existing 
at present in Australia and America. Lingula was once abundant all over the globe; it now only 
Jives along iiortious of the American 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 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 larvm, as compared 
with the hoinogeneousness 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 their early rather than later stages. 
This incongruity between the larval and adult stages, then, was jirobablymost marked in the 
periods before the Quaternary, while since then there has been divergence. We have some reason 
to suppose that the families of Noctuidm and Geometridm, so numerous in species, were largely 
ovolved 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 Avas 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 imocess 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 Euroj)e, we should expect to discover among them 
representatives of the Cochlioiiodidm, of the Attacine Saturniidje, Ceratocampidse, and Notodou- 
tidm, now charactei'istic of North and South America or of the troi)ical regions of Asia and pei^haps 
of Africa. 
Among the Notodontida^ the Ileterocamindm, for example, now confined to eastern North 
America, Central America, and western South America, may have flourished in Europe contem- 
lAoraneously with the sequoia, magnolia, liquidambar, gum tree, and other existing types of 
vegetation now extinct in Europe. Although Macrurocami^a is an American genus, some form like 
it may have existed in Europe, from Avhich the European Cernrinw may haA'e evolved, unless the 
type migrated 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, i^erhaps as the result of the cold of the Glacial epoch. 
