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It is part of the process of the formation of new species and is related to subspecific differentiation.

Geographic considerations alone are responsible for our use of the subspecific name mexicanus; we do not know the morphological features which distinguish mexicanus from other named subspecies.

In preparing the original description of R. c. seclusus, known only from specimens in the summer pelage, comparison of color was made with only the winter pelage of R. c. chrysopsis and it was pointed out that the differences noted in color between the two subspecies might be seasonal rather than subspecific.

Remarks.—From the synonomy at the beginning of this account one may note that the animals here ascribed to this subspecies have had nearly as many subspecific names applied to them as there have been investigators who have written about them.

Although each of the previous writers had but a small amount of material upon which to base his opinion, the diversity of opinion as to subspecific status bespeaks the instability of these animals.