26 



NA TURE 



[November 13, 1902 



part of this great monograph still unfinished. Three more 

 years were required before the work was completed by 

 F. D. Godman, who speaks of the large amount of help 

 rendered him by the skill and energy of G. C. Champion, 

 and the value of Dr. Holland's excellent book, which 

 would have been a still greater assistance had it become 

 available at an earlier date. 



The immense development of the Central American 

 Hesperiida?, that difficult group which so long delayed 

 the completion of the work, will be appreciated when the 

 number of species, upwards of 550, is compared with 

 the 178 of the New-World segment of the northern belt 

 and the 66 of the Old-World segment. The study of 

 the Pamphilinae was a special cause of delay, and not 

 only here, but in the whole family, an examination of the 

 male genitalia, requiring the preparation of immense 

 numbers of dissections, was found to be necessary. 



" In Thanaos several of the species are absolutely in- 

 separable by external peculiarities, but markedly different 

 in their genital structure." 



Such cases are remarkable and interesting, and range 

 with those in which the males, as in many Enploeina?, 

 have not hitherto been separated except by the wide 

 difference in secondary sexual characters, viz. the con- 

 spicuous "brand" on the wings. That species should 

 be separable only by the characters directly or indirectly 

 associated with the reproductive system of a single sex, 

 while these sole differentiating criteria are strongly 

 marked and evident, suggests the possibility of sexual 

 dimorphism, rather than specific distinction, as an inter- 

 pretation. Carefully conducted breeding experiments 

 upon a few well-chosen examples would be well worth a 

 trial, and would speedily decide beyond the possibility of 

 doubt as to the interpretation of an immense mass of 

 interesting facts of which the discovery is recorded in 

 this monumental work. 



Turning to the bearing of this section of the " Biologia " 

 upon the broad principles of geographical distribution- 

 one of the most important aspects of the whole work— it 

 becomes necessary first to define precisely the extent and 

 limits of the term Central America as employed by the 

 editors. The area will be best understood by the 

 enumeration of the following eight component districts : — 

 North Mexico, South Mexico, British Honduras, Guate- 

 mala, Honduras, Nicaragua, Costa Rica and Panama. 

 An excellent account is given of the physical geography 

 of each of these divisions, into which, for the sake of 

 convenience, the editors have separated the whole of their 

 area. They find that the butterfly fauna, which includes 

 many specially modified forms, is mainly a northern pro- 

 longation of the tropical South American, and extends to 

 Mazatlan on the Pacific side and to a little beyond 

 Ciudad Victoria in Tamaulipas on the Atlantic, although 

 some purely tropical genera (Eutresis, Scada, Hetaera, 

 Ithomeis, &c.) do not extend north of Nicaragua, Costa 

 Rica or Panama. The Atlantic slope to as far south as 

 Costa Rica has a more abundant rainfall, a more 

 luxuriant vegetation and an immensely richer butterfly 

 fauna than the Pacific, the difference being especially 

 marked in Ithomiinae, Erycinidae, Thecla, Papilio, &c. 



While the southern forms extend northwards over 

 the coast areas, the Holarctic fauna presses southwards 

 along the high central plateau into Mexico and to some 

 NO. I/24, VOL. 67] 



extent even to Guatemala, where the northern genera 

 Argynnis, Vanessa, Limentis and Grapta are met with, 

 and various northern species of Colias occur. On the 

 higher levels, no strictly Alpine forms are found, the 

 insects above timber-line being mostly stragglers from 

 below, while the highest forests are peopled, as in corre- 

 sponding Andean localities, with species of such genera 

 as Euptychia, Archonias, Catasticta, Pereute, Enantia, 

 &c. 



From the account given above, it is clear that the 

 boundary between the northern belt and southern exten- 

 sion takes, roughly, the form of an attenuated U with its 

 concavity directed toward the north. It has already been 

 stated that no two regional faunas in the world are more 

 unlike than the Holarctic and Neotropical. Their ex- 

 traordinary differences can only be explained by geo- 

 graphical separation for an immense period. At length 

 occurred that " psychological moment " in the organic 

 history of the world when the boundaries fused together 

 and the two contrasted faunas were geographically free 

 to contend and to intermix. The insight of Godman and 

 Salvin led them to investigate the one tract of the land 

 surface of the globe which tells us most of the results of 

 such a struggle. The main conclusion which is impressed 

 by the vast array of facts in the " Biologia" is that stated 

 by Darwin in the " Origin," viz., the predominance of the 

 organic over the inorganic environment of living beings. 

 By the " long results of time," in other words natural 

 selection operating for a vast period, the northern fauna 

 as a whole has been adapted to one environment and the 

 southern fauna to another ; and when the two are at 

 length free to invade and to intermix, very little invasion 

 and intermixture occurs. Each fauna is "an army of all 

 arms," as Rolleston used to express it, strong enough in 

 its own territory to repel the attacks of the other. The 

 metaphor is an exceedingly good one, in that it empha- 

 sises the truth that each species of the whole fauna (and 

 flora) is not only adapted to its inorganic environment, 

 but also to countless other species of the fauna and flora 

 of the same region. The U-like shape of the boundary 

 line between the two regions expresses the fact that the 

 northern forms gain advantage in the cooler higher 

 ground, the southern in the hotter low-lying coast areas. 

 The occurrence of northern genera on high ground 

 towards the south of South America can best be ex- 

 plained by oscillations of level and changes of climate 

 along the north-and-south-running mountain ranges, 

 which have given northern forms an advantage over 

 the southern and enabled bands of immigrants to press 

 southwards until they reached a latitude to which they 

 were permanently better adapted than the Neotropical 

 fauna. Further changes of level and climate would then 

 rapidly ensure the extinction of such species in tropical 

 latitudes, so that the southward-extending bay of the U- 

 formed boundary in the north and the colony of moun- 

 tain forms cut off in the south remain as the only evidence 

 of invasion. 



The impression made upon the north by invaders from 

 the south is doubtless far stronger, chiefly because the 

 northern fauna being so much poorer the successful in- 

 vaders make up a higher proportion of the whole. One 

 marked result of successful invasion is certainly seen in 

 the 17S species of Nearctic Hesperiida; as against the 



