294 DR. H. GADOW ON EVOLUTIOX [Mar. 20f 



separating from C. imimitahilis and guttatus, which are very 



difterent from C. sexlineatus. C, in another direction, turns in 



Michoacan and the Balsas basin into C. mexicavus, which reaches 



• 

 its terminal development in Oaxaca. Lastly, C. hocourti i-epresents 



a third oftshoot from C. comimmis (of. p. 287 and pp. 356-358). 



We have thei'efore to search for another explanation. We postu- 

 late the existence of an indifierent stock, soniewhat like a, combina- 

 tion of G. cle^ypei and C. sexlineatus, with a i-ange from South to North 

 over those pai-ts of the country which at that unknown period did 

 not yet exhibit the pi-esent variety of ph^'sical, bionomic features. 

 Certainly the Western Sierra was there in bulk, but not as it is 

 now\ Then came the physical changes : subsidence of much of 

 the Pacific land ; the development of desert features in the North- 

 west and North ; the transformation of the central lake into a 

 silted-up plain, the central tableland ; the spreading of foi'ests 

 over the Sierra after the volcanic eruptions had subsided, — in short, 

 the assumption of the more recent features of Mexico. 



Hand in hand with these changes went the making of the 

 species, in loco ; and as they spi'ead further upon new ground, 

 they changed further, giving rise to still newei" varieties, races, 

 sul)species, and species, a seemingly endless kaleidoscopic process. 

 This is not a new process ; it was always going on, but we see 

 only the present results, and of the many extinct forms we know 

 naught. 



Broadly si3eaking, there are now three or four main groups. 

 One centres round C. tessellatus, essentially in the North-west and 

 North ; a second comprises the gnlaris- group of the Centre and 

 West ; thirdly, the essentially Southern, tropical deppei-immvtabilis 

 group ; lastly, C. sexlineatus, the least specialised, in the United 

 States. Each of the four geographical regions or main habitats 

 of these gi-oups has its own chai-acteristic features ; they are types 

 of bionomic conditions. 



The greatest number of well-distinguished forms occurs in the 

 Lower Californian district. At least 6 (or 7 with 0. sexlineatus 

 in California) ; 4 of them are insular 



The existence of 3 species on the little Cedros Island is only 

 partly an illustration of the effect of isolation — C. lahialis on Cedros 

 Island, C rubidus only on S. Margarita Island ; maximus, 

 tessellatus, martyr is, and hyjierythrvs occur on islands but also 

 on the neighbouring mainlands, which consequently prove to be 

 veritable refuge-islands, remnants of a formerly larger land- 

 complex. Chaiacteristic of, even peculiar to, this land are 

 C. lahialis and C. hyperythrus, whilst the other species are of 

 tessellatus descent. 



Another centre of great vaiiety is Oaxaca, with likewise 6-7 

 forms which are I'eferable to the deppei and the gularis-communis- 

 mexicanus group. This great diveisity is in concord with the very 

 varied physical features of that State. It is there that the Tierra 

 caliente embraces with an eastern and a western arm the most 

 southern portion of the ^J^ateau, a wedge which is continvied 



