156 Distrihution of Moths of the Sub-family Bistonincs. 



divisions. Therefore, although disunited now, they must 

 have been connected in the past by some common ancestral 

 form. 



In other words, in the old Tertiary continent, some long- 

 lost insect linked them up ; and this species (or it may be a 

 group of species), under the stress of progress, has vanished 

 irrevocably as in the case of the forms linking up the Llama 

 and Alpaca with their old-world relatives the Camel and 

 Dromedary. 



An observation has been made in many families of plants, 

 animals, insects and the rest, that, when identical or derived 

 genera appear to be the common property of both subdivisions 

 of the Holarctic Realm, the fossils almost invariably, but not 

 always, tell us that the Palaearctic form is the older and to the 

 general rule the present is no exception ; Biston is the earlier 

 genus, and this view is borne out by its non-possession of a 

 fovea. 



Nevertheless, one must lay particular stress on the remark 

 that it is not quite universally true that the Palaearctic is the 

 older species, genus, or family. Just as when a wave dashing 

 on the shore recoils, so a spent impulse of a migratory host 

 throws back a counterwave, and this is very probably what 

 has happened to the earlier dog forms entering America ; they 

 have hurled back as a return wave, the true dogs of the genus 

 Canis. Such, too, has been the fate of the first Bistonines 

 passing into the Western Hemisphere, their backwash via 

 the genus Nacophora has been the subfamily BoarmiincB. 

 Returning to the transition between the Bistons and the 

 Nacophoras, from its total disappearance we must consider it 

 to have been a creature of limited distribution in the old 

 Northern Continent or one needing more tropical climates than 

 the Holarctic region of to-day can offer. That the latter is 

 the more correct view we glean from the preference all the 

 Nacophora tribe display for warmer areas— as Biston does when 

 it gets the chance. 



After the separation and final disappearance of the link, 

 Nacophora, acting under the influence of climate, worked its 

 way southward if, indeed, a branch had not already struck 

 out thither in the genus Phceoura and, moreover, other move- 

 ments occurred of which more exact knowledge can be gained 

 from the study of other groups, represented in fossiliferous 

 deposits, which are attached to an arboreal environment like 

 N acophora. 



The coincidence of the range of the subjects of our present 

 essay and that of the Spotted Felicia like the Jaguar {Felis 

 onca), the Puma {F. concolor), Margay {F. tigrina), and the 

 Ocelot {F. pardalis) which, like the Nacophoras, have lost the 

 form connecting them with their Palaearctic relatives may be 



Naturalist, 



