THE LOCUSTS OF THE OLD WORLD. 475 



from countries where the species breeds most. In Europe they would consequently be 

 directed to the west, while iu China they should have a direction ordinarily toward 

 the southeast. 



M. Koppen thinks that the same centrifugal radiation has presided over the scatter- 

 ing of this species beyond its original limits, and that this radiation, propagating in 

 waves, such as we still see produced at the limits of its geographical area, has carried 

 the species from its center of creation or its original country to points "where it is 

 powerless to overcome the climatic conditions or that concurrence of vital forces which 

 are opposed to it. The center of creation or the point of departure of the species will 

 be found, then, in Central Asia. The complete absence of this species on the American 

 continent shows that it only began to exist as a species after the epoch of the separa- 

 tion of America from the Old World. 



M. Preudhomme de Borre adds, " In this study, so interesting, there is one point on 

 which we should insist. It is this : That the observations of M. Koppen tend to confirm 

 the principle of zoological geography, that the area of a species cannot be limited on 

 the map by a simple curve, but between places where the species exist in a constant or 

 normal manner and those where its absence is constant there is always a zone, often 

 very broad, of temporary visitations, which is to the area .properly so called w^hat 

 the penumbra is to the light, within the zone, of which the exterior limit is much more 

 easy to trace than the inner ; this last is subject to continual oscillations, with some 

 undulatory movements, dependent on the centrifugiil or expansive tendency of the 

 species, and from the resistance which opposes it, and external circumstances, and evi- 

 dently also the tendency of other species to spread out, with which it carries on a 

 struggle for existence in endeavoring to maintain itself on an earth where the chances 

 are divided, and even vary from year to year. M. Koppen has thus been enabled to 

 figure on his chart three lines, as I may for the present call them, and the intermediate 

 line represents the exterior actual limit of these oscillations of the true frontier line 

 of Pachyiijlus migraiorius ; their amplitude may vary from two to four degrees." 



The last thesis of M. Koppen that I shall draw attention to at this time, namely, 

 that the absence of Pachytylus migraiorius in America should prove that the species 

 exists only as a species, since the separation of the two continents toward the north 

 pole, seems to me scarcely necessary. A mere glance at the map which represents the 

 area of distribution of this locust allows us to afifirm without hesitation that that view 

 is impossible. It is evidently not one of those species which we may call circuviboreal 

 anteglacial, because their presence in two forms (races, varieties, or species) on each 

 continent indicates that they have had a common origin, a single area at that epoch, 

 anterior to the glacial period, when the two continents were reunited in the Arctic 

 zone by a bridge, so to speak, that is, a continuity of land, in conditions of climate 

 which should allow the existence at that latitude of a fauna which only at present 

 exists much farther south. The source of those species dispersed by the glacial period 

 does not now probably exist in its integrity ; but the two races confined, one in America, 

 the other in the Old World, having undergone slow modifications each on its part, are 

 to-day very analogous species, but as distinct by their external characters as by their 

 separate geographical area. 



Nothing like this applies to Pachytylus migraiorius ; it is one of those species which 

 may be called equatorial postglacial ; its expansion toward the north has been posterior 

 to the glacial period, which would then have opposed it ; and it can have no afiflnities 

 in the New World, but degrees of consanguinity much farther removed than those 

 unite tl^e circumboreal species of the temperate zone. Thus, if, as some think, the 

 northern hemisphere tends actually to retrograde toward a new period of cold, the 

 Pachytylus migraiorius is destined to see its area also retrograde toward the equator, 

 and perhaps some day the western and eastern parts of this area may be completely 

 disjointed, and, following this separation, its posterity may be so modified by isolation 

 as to form two distinct species, as has occurred to circurapolar species. 



In the discussion which followed, M. de Selys-Longchamps speaks of 

 the difficulty of separating Pachytylus migratorius (Linn.) and cinerascens 



