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Show 288 DR. II. GADOW ON EVOLUTION [Mar. 20, best they are subspecies, if not local races, or, worse still, only pattern-varieties. In short, we have here two forms hi the actual process of evolution, which require only the accident of a physical separation belt- which of course would not alter the remaining individuals-to give them the standing of local races, but scarcely of subspecies on account of the slight structural differences, and this because they are still in the process of making! It is fairly safe to consider the var. immutabilis as closely allied to C. deppei, perhaps as a larger form evolved from a more generalised clan of C. deppei. On p. 319 the question is discussed whether true links still exist between them, but none have been found. It is therefore concluded that C. deppei and C. immutabilis being practically coterminous in their wide range, their differentiation from the hypothetical common stock had proceeded far enough to turn them into "species," implying the disappearance of links. In other words, these two forms, concerning each other, are no longer in the act of being made *. This may mean either that their divergence dates back a longer time, or that they have divided the ground between them sufficiently well, leading lives too different for competition, and too diverse in the ensuing reaction upon the surroundings, so that the differentiation has proceeded more rapidly. The facts that C. deppei inhabits also the Atlantic hotlands, where it meets the C. guttatus (from which it is structurally and in pattern more widely removed than from the C. immutabilis), and further, that C. deppei has such an enormous range southwards into South America, these circumstances rather favour the assumption that C. depj)ei is an old form and that the evolution of C. immutabilis is of an older date than its splitting into the present striped and spotted or Pacific and Atlantic races. Present species are older than subspecies, and these are older than their present races. On p. 305 the very pertinent question is discussed whether the small C. deppei is always separable from the equally small C. sexlineatus, the least differentiated, the most primitive of the whole genus, of which, by a fortunate accident, it happens to be the type. AVe there succeeded in singling out some specimens of C. sexlineatus from Sauz near Chihuahua,, and of C. deppei from South Guerrero, which apparently are not separable; but we had to explain these as cases of convergent development, or, let us say, as due to the coincidence of the variations of all the characters employed. Some valid reasons were given to show that these Guerrero clans are local varieties of the other surrounding C. deppei. The argumentation seems satisfactory, but it would have been far less so, if these convergent lizards had been taken in neighbouring districts, instead * The differences are, however, sometimes so small that, if, for instance, the Cajones (text-fig. 81E) or the Miahuichan specimens {cf. p. 326) were the only representatives known of C. immutabilis, we should unhesitatingly treat them as a subspecies of C. deppei! |