MEMOIRS OF THE NATIONAL ACAL)E:\IY OF SCIENCES. 51 



If we realized liow arbitrary onr zoolojiicai classitications arc, especially the categories we call 

 Npccies, genera, families, and orders; if we could erase from our hooks and from our minds these 

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

 the prejudices resulting from our often untimely and hast}' attcMnpts to — without ade(|uate knowl- 

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

 fications, 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 merelj' 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 are 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 frecjuent 

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

 discontinuous steps as well as by 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 Ijranches of the phylogenetic tree or sudden 

 departures from the main stem or trunk, branches which often are the result of eV'Olution by 

 atrophy, become bent downward and backward, as in Saturnides, the bombyciform types of other 

 lepidopterous families, or as in ecto- or eudo-parasitcs of other orders, classes, and phjia. They 

 are so frequent that we miust consider them as the necesnary 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 types 

 would be unable to exist. 



XIV. THE GEOGRAPHICAL DISTRIBUTION OF THE CERATOCAMPIN^. 



(Fiij. 6 on p. 62; also iiiapx I to IX.) 



This subfamily' is entirely confined to the western hemisphere, and practically to the 

 tropical and subtropical belt of the two Americas. 



The center of origin was most prol)ably the region extending from Brazil to the Isthmus 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 \>\ 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 su))region; one species of Adelocephala {A. cdliinJua), however, is recorded from 

 Colombia and Cltheronia eminent from Loja, Ecuador. While the absence of any other forms 

 in these countries may be simply due to lack of, extended exploration, it is quite 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 iidiabit the tropical belt 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 City of Mexico, 

 but none have yet been found in the dry regions to the north and northwest. 



