294 DR. H. CtADOW ON EVOLUTIOX [Mar. 20^ 



separating from C. imimitabilis and guttatits, which are ver}' 

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

 Miclioacan and the Balsas basin into C. mexicanus, which reaches 

 its terminal development in Oaxaca. Lastly, C. hocourti re'presents 

 a third offshoot from 0. communis (cf. p. 287 and pp. 356-358). 



We have therefore to search for another exj)lanation. We postu- 

 late the existence of an indiflerent stock, somewhat like a. combina- 

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

 over those parts of the country which at that unknown period did 

 not yet exhibit the present variety of physical, 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 develojDment 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 forests 

 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 spread further upon new ground, 

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

 subspecies, and species, a seemingly endless kaleidoscopic process. 

 This is not a new j^rocess ; it was always going on, but we see 

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

 naught. 



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

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

 North ; a second compi'ises the gularis-gron]) of the Centre and 

 West ; thirdly, the essentially Southern, tropical dej)pei-immutahilis 

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

 States. Each of the four geographical regions or main habitats 

 of these groups has its own characteristic features ; thej' are types 

 of bionomic conditions. 



The greatest number of well-distinguished forms occurs in the 

 Lower Calif ornian district. At least 6 (or 7 with C. sexlineatus 

 in California) ; 4 of them are insular 



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

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

 Island, C. nbbicliis only on S. Margarita Island ; onaximits, 

 tessellatus, ma7'tyris, and hyperytlirvs 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 G. hyperythrus, whilst the other species are of 

 tessellatus descent. 



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

 forms which are ]:'eferable to the cleppei and the gularis-communis- 

 mexiccmus group. This great diversity is in concord with the very 

 varied physical features of that State. It is there that theTierra 

 caliente embraces wath an eastern and a western arm the most 

 southern portion of the plateau, a wedge which is continued 



