1906.] IN MEXICAN LIZARDS. 285 



permutations is enormous, at least theoretically, but in reality it 

 cojnes to pass, that, owing to some occult law of correlation, certain 

 combinations do not occur. These give us a clue as to the specific, 

 subspecific, &c. value of the items employed. For instance, in the 

 whole genus of Gnemidopliorus the a liriori obvious combination of 

 large postei-ior arm-scutes with only 3 supraoculars does not occur, 

 except as individual freaks or true abnormalities. A well-scutellated 

 forearm is mostly associated with a large-scaled collar and with 

 4 supraoculars, perhaj)s beca.use the prevailing bionomic conditions 

 favour a strong lepidosis ; but where the yenms loci favours 

 small scales, the completely granular forearm is coupled with a 

 small-scaled collar (e. g. in the tessellatus- gvow^, text-fig. 64 A) ; or 

 the collar is in an unstable condition, the scales decreasing in size 

 towards the sides of the collar and interspersed granules are fre- 

 quent on the edge (e. g., in G. depjjei, C. sexlineatus, and G. comrnvMis 

 austrcdis, text-fig. 65 D, E). It is then a question which part 

 initiates the change, and which parts follow suit, or, may be, are 

 not allowed to yield to the new tendency. For " not allowed " we 

 may say " overruled by natural selection." 



Unfortunately we know next to nothing about the advantages 

 of these features. Broadly speaking, large forearm-scutes are a 

 feature of the high plateau and of the mountainous districts, but not 

 of forests, grassland, or sandy soil. I rather suspect that arm-scutes 

 are connected with a rough ground, physically rough in its detail 

 composition, such as is produced by the rubble of volcanic and 

 limestone formations. The same agency may be applicable to the 

 collar, but not conceivably to the supraoculars. 



Concerning the colour 2Kittern. A spotted garb, light spots upon 

 darker, uniform ground-colour, seems to be the effect of forest or 

 bush life upon an originally sti'iped creature ; for instance, G. immu- 

 i«6i7is compared with6\ guttatus, and G. bocourii with C. mexicanus; 

 but it is also the reaction of a life on the open, periodically droughty 

 plateau, e. g. the strongly spotted G. conmiunis. This seems con- 

 tradictory, but the tertium comparationis, the moving agency is 

 the monotonous light, whether this be due to the abundance of 

 broken shade, the subdued light in the forest, or the absence of 

 shade in the open under a glaring sky. In neither case can the 

 organism retain the stripes ! Moreover, that unknown influence 

 which causes the appearance of pale field spots has thereby intro- 

 duced a new element, witness the behaviour of these spots during 

 their growth so that a longitudinally striped pattern may be con- 

 verted into one of transvei'se stripes. However, these questions 

 have been dealt with, tentatively at least, in Proc. R. S. 1903. 



The so-called systematist wants above all to label and shelve his 

 specimens ; his beau ideal is a good species, and his ambition as 

 many of them as possible. His chief anxiety is to point out the 

 differential characters. Every individual belongs, in his idea, 

 to a perfectly definable assembly, the ultimate boundaries of which 

 enclose the "species." There ai'e also others, not " splitters," but 



