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Show 1906.] IN MEXICAN LIZARDS. 285 permutations is enormous, at least theoretically, but in reality it comes to pass, that, owing to some occult law of correlation, certain combinations do nob occur. These give us a clue as to the specific, subspecific, &c. value of the items employed. For instance, in the whole genus of Cnemidophorus the a priori obvious combination of large posterior 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, perhaps because the prevailing bionomic conditions favour a strong lepidosis; but where the genius loci favours small scales, the completely granular forearm is coupled with a small-scaled collar (e. g. in the tessellatus-group, 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-quant on the edge (e.g., in C. deppei, C. sexlineatus, and C. communis australis, 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 pattern. A spotted garb, light spots upon darker, uniform ground-colour, seems to be the effect of forest or bush life upon an originally striped creature ; for instance, C. immutabilis compared with C. guttatus, and C. bocourti with G. mexicanus; but it is also the reaction of a life on the open, periodically droughty plateau, e. g. the strongly spotted C. communis. This seems contradictory, 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 introduced a new element, witness the behaviour of these spots during their growth so that a longitudinally striped pattern may be converted into one of transverse 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 are also others, not " splitters," but |