The present article has for its purpose to state the principal facts and to make some immediate deductions relative to the individual variability of plumage in some Tetraonidae.

On the basis of collections made during many years by three persons: the sportsman W. Andriewsky (1850—1880), the well known naturalist Theodor Lorenz (1870—1909) and the writer (1896—1936), these collections being derived from huge shipments of game consigned to the two capitals (Moscow markets alone received no less than a million pairs Black Grouses per year...), the Darwin Museum in Moscow has succeeded in obtaining a unique material on the individual variability of Grouses and especially of the common Black Grouse — Lyrurus tetrix L.

Confining our task to a consideration of symmetrical variations in colours only, we may state that the last named bird displays ten different varieties in the plumage of the cock and a double number in that of the hen.

The chief variations of the cocks: 

Those of the hens: 

As it is evident from the given list, only a small part of the varieties of males can be quite exactly paralleled with those of females (as Var. albiventris, lanceolata, marginata). Part of the varieties can be but conventionally referred to both sexes (e.g. the varieties — brunnea, fumosa, chalybaea of cocks may be said to correspond to varieties flavo-albicans, flavescens, ochracea, pallida of hens).

A considerable part of the varieties of the plumage of females is too closely connected with the peculiarities of the «normal» plumage of the hen and can in no way be compared with the variations of males.

Being an excellent example of the «Pluripotenz» phenomenon (Valentin Häcker), this multiplicity of potentialities in plumage-colour shows that the prevailing normal dichromatism of the Black Grouse is in fact nothing but one out of a great number of equally well expressed and just as stable colours (partly mono- and partly bi-chromatic) which latently belong to this bird and are in fact, equivalent to its normal plumage.

Passing over to the closely-related phenomenon of «Paripotenz» (Val. Häcker) and the study of some other representatives of Tetraonidae, it is possible to trace the availability of «Homological series» (Vaviloff) for «abnormal» individuals of the Capersaillie, Tetrao urogallus, (Var. alba and chalybaea in the cock and the Var. ochracea, subalbida, pallida and lactea in the hens), the Hazel Grouse, Tetrastes bonasia L. (Var. opaca) «griseiventris», Menzbier (Var. brunea, chalybea, ochracea, albiventris, lanceolata) and partly also in the Willow Grouse, Lagopus albus (Var. lanceolata).

It is most remarkable that it has been possible to ascertain similar homological series — true in part only — with regard to certain parts of the plumage also in the representatives of the fam. Phasianidae, and not only in the pheasants (as for instance, in the well known Phasianus colhicus subalbidus!) but also in the domestic fowl.

At any rate it is possible to say that a number of characteristic plumage colours and patterns typical for different breeds of poultry can be easely brought back to the principal colour varieties of the Black Grouse, as will become clear from the following examples:

Bearing evidence to historical development, or, in other words, to the process of evolution, but certainly throwing no light on the genealogical problem (the consecutiveness in which different varieties arose) both these phenomena namely the Pluri- and Paripotenz of colours in the Tetraonidae clearly testify in favour of the non-adaptive character of most varieties of the Black Cock. Remaining for the most part in latent genotypical reserve and appearing but very rarely, from time to time, through special crossing these «abnormal» colours can survive only occasionally, when they are linked up with an adaptative plumage of the female, as in the case of the «normal» black variety (Var. splendens) of this bird.

The very fact of the potential existence of a great range of plumages in the Black Grouse allows of finding a more critical line of approach to the problem of «adaptation» and is useful in throwing some new light on a fundamental question of general biology namely the problem of the true relation between the genotypical and the historical variability of organisms.