MEMOIRS OF THE NATIONAL ACADE:\1Y OF SCIENCES. 51 



If we realized liow arbitrary our zoological classitications are, especially the categories we call 

 species, genera, families, and oi'ders; if we could erase from our books and from our minds these 

 artificial pigeonholes into which groups of individuals are thrown, and could divest ourselves of 

 the prejudices resulting from our often untimely and hast}' attempts to — without adecjuatc knowl- 

 edge of the morphology, ontogeny, and life conditions of organisms — frame our ephemeral classi- 

 tications. we should realize that the secular growth of organic forms, to which we give the name 

 of evolution, is all of a piece with the causes, modes, and results of growth of any individual. 

 What we call primitive generalized forms and specialized forms are merely such stages as we 

 happen to have discovered, or (taking into account the fossil forms) fragments of defective series 

 of forms in process of evolution. Could we see the whole series arranged in the order of their 

 evolution we should realize that in the creation of any phylum or group of blood relations the 

 phylogenetic stages or steps ai-e in the long run, or throughout the whole course of evolution, 

 the result of a process of gradual, slow, secular moditications, with accumulated phases, which 

 appear to us as sports or mutations, and to which process we give the name of discontinuous 

 evolution. There is not an uninterrupted, progressive, ascending series, but there are frequent 

 pauses and backward steps or reversions. Evolution has gone on both by progressive and by 

 discontiiuious steps as well as Ijy atrophy. There are often no intermediate forms or stages. 

 Rapid or saltatorial evolution may be compared with the sudden acquisition of characters seen at 

 the time of molting in insects, Crustacea, etc. 



These phases or aberrations, often forming side branches of the phylogenetic tree or sudden 

 departures from the main stem or trunk, branches which often are the I'esult of evolution by 

 ati-ophy, become bent downward and backward, as in Saturnides, the bombyciform types of other 

 lepidopterous families, or as in ecto- or endo-parasites of other orders, classes, and phyla. They 

 are so frequent that we must consider them as the necessary and normal or natural results of 

 changes in the environment, leading to change of habit, station, food, and means of locomotion, 

 the tinal result being adaptation to certain niches, corners, stations, and hosts, where normal tvpes 

 would lie unable to exist. 



XIV. THE GEOGRAPHICAL DISTRIBUTION OF THE CERATOCAMPIN/E. 



( Fkl. fj on p. 63; also iiKipx I to IX. ) 



This subfamily is entirelv confined to the westei'n hemisphere, and practically to the 

 tropical and sul)tropical belt of the two Americas. 



The center of origin was most probably the region extending from Brazil to the Isthnuis of 

 Panama. At present the group extends over the greater part of tropical and subtropical South 

 America or the Brazilian subregion of Wallace. Several species pass south of this region, as 

 limited by him, into Paraguay and the valley of the La Plata in the Argentine Republic. On the 

 other hand none has yet been detected in the region of the headwaters of the Amazon, nor in 

 Bolivia or in eastern Peru, and none have been recorded from Venezuela and the West Indies 

 or Antillean subregion; one species of Adelocephala {^4. ciilitiiihiu). however, is recorded from 

 Colombia and Citheronia eminens from Loja. Ecuador. While the absence of an_v other forms 

 in these countries may be simply due to lack of extended ex])loration, it is (piite the reverse with 

 the ceratocampinine fauna of Central America. 



In its general characteristics the Central American fauna repeats that of the Brazilian 

 subregion, as will be seen by the following lists. The greater number of Central American 

 species inhabit the tropical Ijelt along the eastern coast and on the Pacific coast .south of north 

 latitude 20"^, and a few occur on the temperate plateau of the region about the Citv of Mexico, 

 but none have vet been found in tiie di'v regions to the north and nortliwest. 



