VII. HEREDITY OF COLOUR (continued) : page 115.
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HEREDITY OF COLOUR - continued.
Various Specific Phenomena in Colour-Inheritance. Relation of Colour to Hoariness in Stocks. Miscellaneous Cases. Colour of a Special Part controlling that of other Parts. - Summary and Discussion. - Subtraction - Stages. Again and again in tracing the genetic properties of colours in animals and plants we encounter the phenomenon of a specific connection between certain colours and their modes of hereditary transmission on the one hand, and various apparently distinct physiological properties on the other. Colour, which systematists have often spoken of as one of the superficial or impermanent properties of organisms, seems thus to be bound up with fundamental phenomena of chemical economy. To treat this part of genetics with any fulness is not yet possible. As an illustration may be mentioned the curious result discovered by Miss Saunders in Matthiola, using the varieties known as "ten-week Stocks." These may be either "hoary," viz. covered with branching hairs forming a tomentum, or glabrous and destitute of hairs. When the hoary are crossed with the glabrous, hoariness is an ordinary dominant, giving 3 hoary : I glabrous in F2.
But when certain glabrous strains are crossed together the F1 form is hoary, reverting to the primitive type. This reversion never occurs when any of the many red or purple varieties are crossed together, but is universal when any of them are crossed with either the white or the cream-coloured glabrous strains. The purples and reds owe their colours to the presence of coloured sap. This coloured sap is not present in the whites, nor in the creams, whose colour is due to the existence of yellow plastids in the cells of their petals.
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F2 from the cross, for instance, of purple x white contains sap-coloured and non-sap-coloured plants, and of these some are hoary and some glabrous. But none of the plants whichcome without coloured sad have any hairs on their leaves. A consideration of the case shows that the factor for hoariness is really introduced by the while-flowered glabrous plant, and that the glabrousness is due to the inability of the hoariness-factor to make the hairs grow in the absence of the factors for sap-colour. The facts may be represented thus, C and R representing, as before, the factors for sap-colour, and H the factor for hoariness.
[Table not reproduced in this version]
If cream glabrous be substituted for white glabrous the result is the same so far as sap-colour and hoariness are concerned, and in F2 only those plants can be hoary which also have coloured sap.
Finally when cream and white glabrous types are crossed together, F1 is purple and hoary, thus showing reversion in colour, owing to the meeting of the two complementary factors C and R, one coming in from the cream and one from the white ; and also reversion to hoariness because the hoariness-factor was really present all the while in both the cream and the white types, but was unable to show itself because one of the sap-colour elements was absent in each type. The heterozygosis of the two types brings together all the three elements C, R, and H, so the F1 plants are both coloured and hoary.
The reason why hoary-leaved plants are never produced by crossing two types possessing coloured sap is at once apparent. For if the factor for hoariness were present in these types, they would be hoary.
As may well be supposed the disentangling, of these results was a long and tedious process. The occurrences seemed at first contradictory, but after it had been ascer- [ascertained]
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tained that each kind of family was produced with regularity as the consequence of a particular kind of union, the work of bringing all these into one analytical scheme was only a matter of time. Much still remains to be done before the analysis will be complete for Stocks in general. For example, among the Brompton Stocks races occur which are hoary though devoid of sap-colour, and as yet we are not aware what condition or factor exists which there enables the hoariness-factor to assert itself.
In cases like these we get glimpses of the strict specific rules which govern the genetics of pigmentation. In the Sweet Pea again we have found that both variation in the pollen-shape and in the structure of the standard petal are closely related with the distribution of the factor which turns the colouring matter purple. There is every hope that in our further analyses these apparently trivial phenomena will serve as indications of the underlying processes.
Apart however from these curious inter-relations between colours and structural peculiarities, there are several remarkable specific phenomena to be seen in the genetic behaviour of colours. Of these some examples may be given as incentives to future experiment. General rules regarding colour-inheritance are scarcely to be expected as yet, for very little is known of the pigments of either animals or plants. Beyond the fact that albinism has always been found to be recessive to colour in both animals and plants no general proposition can be put forward with confidence. We believe also that yellow chromoplast-colour is always recessive to white or colourless chromoplast-colour, though the cases studied are not numerous enough to justify a general assertion. Stocks, Sweet Peas, Swedes and Turnips, Verbascum,may be cited as plants following this rule and no clear exception is yet known.
Mr Arthur Sutton tells me it is well known that when Swedes are being grown for seed, Turnips must not be allowed to flower near them, but that in growing turnip-seed, no injury is done by the presence of flowering Swedes. The meaning of this is now clear. The Swedes are in general yellow-fleshed, their colour being due to yellow plastids. Turnips as a rule are white. If therefore the
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pollen of Turnips is carried by insects to the Swedes, the hybrid thus produced will be white-fleshed, and consequently attract attention, spoiling the uniformity of the crop. But as the white of the Turnip is a dominant, no visible effect is produced even though Swede pollen is brought by the insects to the Turnip flowers, for F1 is white. In F2 of course yellow Turnips would appear, but as the roots are almost always eaten off, this result is scarcely ever reached by the farmer.
It might be expected perhaps that the blue and purple colours in flowery would always be dominant to the reds, but this is not so. In Stocks, Sweet Peas, Peas, and Salvia the purples are dominant. Probably the same is true for the blues of Delphinium, Cineraria and a good many more, but when we come to Primula Sinensis we find blue a recessive to the reds and magentas. Doubtless the chemistry of the blue pigment is there quite different.
In Solanum and Atropa black fruit is dominant to the yellow fruit, but in Bryonia the red fruit of B. dioica isdominant to the black fruit of B. alba.This paradox has been elucidated satisfactorily by Miss Wheldale. She tells me that the nature of the distinction between the two types is quite clear. In Atropa the black colour is due to the presence of a dark purple anthocyanin which like other pigments of the same kind is dominant to its absence. In Bryonia alba the black colour is caused by the presence of a little carotin in plastids, together with green chlorophyll undecomposed. In the red-berried Bryonia dioica the chlorophyll is decomposed (just as it is in the cotyledons of yellow-seeded Peas) and much carotin is present. Consequently, as may be expected, the presence of the decomposer of the chlorophyll is a dominant, as also is the abundant development of the carotin, and thus the black colour of the fruits is recessive to red.
In Rabbits, as has been stated above, yellow is a recessive to black, while in Mice it is a dominant.
The yellow varieties of many red Lepidoptera (Zygaena, Arctia, &c.)are presumably recessive †, and the same is
* From Mr Sutton's experiments (262) it seems however that F1 is sterile.
† Proved for Callimorpha dominula. See p. 44.
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apparently true of the yellow fruits, as compared with the red fruits of their corresponding types, and of the yellow flowers of some Composites (e.g. Gerbera)*.
In many types of flowers, e.g.Stocks, Primula, Sweet Pea, the very dark and more fully-coloured varieties are regularly recessive to the less dark types, whether purple or red. The same will almost certainly be proved for Cyclamen, Rose, Hollyhock, Dahlia, Carnation, Sweet William, and many more. In Antirrhinum Miss Wheldale finds that among magentas the darker are recessive to the common colours, but among the crimsons or reds the darker are dominant to the lighter.
The difficulties which preclude general statements in regard to the genetic relations of melanic types among animals have been illustrated in much that has gone before. The loose description "melanic varieties," common in the writings of systematists, covers a number of phenomena essentially distinct. For example there are melanic forms which owe their greater blackness to the presence of some dominant factor responsible for a greater deposit of black, or at least dark, pigment. In Fowls, for instance, black is at least partially dominant over the bankiva colour which fanciers call "Black-red." The dark brown variety called "Brown-breasted" is similarly a dominant. In Pigeons, as Staples-Browne has proved, black is dominant to the blue of the wild type. In the Horse the presence of black, as in bays and browns, is dominant over the absence of blacks as in chestnutst†.
On the contrary in Rabbits, Rats, Mice, &c. the black variety is produced by the omission of the agouti-factor, G,from the wild type, and black thus is apparently a recessive. Even here however the presence of black pigment is dominant to its own absence. It would be interesting to know to which group the Cat belongs.
In Insects again no rule of universal application to
* That in Tomato yellow fruit is recessive to red was established by Hurst (160, p. 115). The case of Gerbera is given on the evidence of crosses made by Mr R. I. Lynch between red Gerbera Jamesoni and the yellow-flowered variety "Sir Michael."
† The genetic relation of the totally or self-coloured black to the other horse-colours is not yet known.
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melanic varieties can be given. Of the melanic varieties of Moths which have been tested several are apparently dominant, more or less, to the normal or non-melanic types ; but in the black Chrysomelid Beetle, Lina lapponica,investigated by Miss McCracken, the evidence shows plainly that the uniformly black type was recessive* to the normal which has black only in the form of spots. The common melanic varieties of the 2-spot Lady bird (Coccinella bipunctata)are probably also recessive to the ordinary red type.
The experiments of Standfuss interpreted according to the Mendelian system show that the dark variety lugens isdominant to the ordinary fulvous yellow type of Aglia tau (a Saturniid Moth). This case comes up for consideration in some detail with reference to the heredity of Sex (q.v.).
In Silkworms a melanic variety of the moth is an imperfect dominant to the normal, pale-coloured moth, giving a blend-form in F1 (Coutagne, 83).
As regards the colour of the silk some interesting results have been obtained. Yellow silk was always. found by Toyama (268) to be dominant to white and this result was obtained by Coutagne in certain cases†. For example the yellow race called "Var" was dominant in silk-colour to the white Japanese race used by Toyama and to the white "Bagdad" used by Coutagne ; but Coutagne found the same yellow to be recessive to the white of two French races with which he experimented. Presumably this distinction is due to some idiosyncrasy on the part of the whites, analogous to what has been seen in fowls and Primula, but as to this nothing is known.
In crossing a yellow Siamese race with a white Japanese race, Toyama obtained a resolution-effect in F2. Yellow x white generally gives yellow F1 with 3 yellow : 1 white in F2 ; but in this special case there were two new forms in F2,a pale pinkish yellow, and a greenish white. This latter white could not always be satisfactorily distinguished from the pure whites, so the F2 family has to be taken as 9 : 3 : 4.
* There is a possible complication in this case.
† Toyama states that the colour of the silk always corresponds to that of the abdominal legs of the larvae, and consequently it is not necessary to rear all the larvae up to the spinning stage in order to ascertain the colour of their cocoons.
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Observation, for example, gave 70 : 21 : 36, the expectation being 72 : 24 : 32. In certain other cases this resolution-effect did not occur, though from analogy it might have been expected. Toyama regards the distinction as due to differences in the whites used, but it seems not impossible that it was really the yellows which possessed individual differences in this case. In either event there are difficulties to be faced, and on the evidence it is not clear which account is actually the more probable.
There are some illustrations of a principle by which the colour of one part of the organism may lima or control the possible colours of other parts.
In animals it is fairly certain that the eye-colour may act in this way, certain coat-colours being produced only if the eye be black, and others only if the eye be chocolate, but the facts are still somewhat obscure.
If the stem of the Chinese Primula be green and not red the deeper flower-colours cannot be developed in self-coloured types. Across with a red-stemmed type, however pale in flower-colour, at once reveals the presence of the factors for the deep colours if they are there.
On the contrary, the white-edged types,such as Sutton's "Sirdar," though their flowers may be of a deep shade of purple or red, appear exclusively on stems which are green throughout except for a development of red colour at the collar or extreme base of the petioles. Such " Sirdars" cannot exist on a wholly-coloured stem. The stem may be parti-coloured in Primulas though the flower is wholly-coloured, but these special types of parti-coloured flower can only occur on a parti-coloured stem. In the F2 series it is curious to see these deeply coloured, white-edged, flowers on stems apparently green, while none of their green-stemmed sisters with self-coloured flowers can bear a flower darker than pale salmon-pink.
Another striking example of the same phenomenon is to be seen in these Primulas, with the difference that there the want of a particular colour in the critical or "controlling" position is due to the dominance of a negative character, not to the absence of a complementary one. Certain deep red spots occur in some varieties, e.g.Sutton's "Crimson King"
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(a fine dark red), on the petals just external to the yellow eye (see Plate VI). These spots are never formed unless the stigma is red.When such a type is crossed with one having a green stigma, F1 has a green stigma and no spots on the petals. In F2 there are of course some with green stigmas and some with red, some with spots and some with no spots. But the distribution of these two characters shows that the combination green stigma + spot on petals does not occur. The stigma may be red though no spot be formed, but if the stigma be green, the spot is absent, though the factor for it may exist in the individual.
Formerly such cases might have been regarded as examples of "correlation," but that term is only applicable to them in a loose and quite incorrect sense.
Nothing so fully demonstrates the fundamental significance of colour in the economy of plants and animals as the strange series of phenomena that have been discovered in regard to the complex inter-relations between the genetic behaviour of certain kinds of pigmentation on the one hand and certain structural features on the other. In the chapters dealing with gametic coupling and with the heredity of Sex it will be shown that not only the factors governing structure, but also the factors which are the ultimate cause of sexual differentiation, may be distributed among the germ-cells according to systems which are modified and ordered in inter-dependence on the distribution of the factors for colour.
Summary and Discussion of the Evidence as to the Genetics of Colour and Colour-Patterns.
Since we have abundant proof that the development of colour and even of particular colours may be bound up with other features of morphological or physiological importance, it is clearly impossible to regard the genetics of colour-characters as apart from the rest. A summary of the chapters dealing with that subject will nevertheless be useful at this point, and with this may be combined a brief discussion of essential points.
In many animals and plants colour has been shown experimentally to behave as if due to a single allelomorphic
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factor. In three cases among plants (Sweet Pea, Stock, Orchids) however we already know that the production of colour (sc. sap-colour) requires the fortuitous concourse of two complementary factors which have independent distributions in gametogenesis, and individuals lacking either of these factors are completely devoid of colour.
In the light of this discovery we naturally ask whether it is not probable that the sap-colours of plants in general may not in reality be produced by pairs of complementary factors. It is tempting also to speculate on the possibility that the colours of animals may have a similar nature. At present however the objection holds that in no species of animal have two pure albinos been found to produce coloured offspring when mated together, and the F2 ratio from the cross albino x coloured is always 3 coloured : 1 albino, never 9 : 7. But it may be suggested with great plausibility that this simply indicates that every individual, coloured or albino, contains one of the two factors, and the question whether colour is a single-or a double-factor character remains undecided. If a variation were to occur by which the supposed common factor was omitted from the composition of an albino, albinos bearing respectively each of the two factors could be raised, and nothing would then preclude the production of coloured individuals by crossing the two sorts of albinos together.
At first sight some of the facts related in regard to fowls seem to supply evidence of this kind. White Silky x a recessive white strain produces F1 fully coloured. But neither parent is an albino in any strict sense, for both have, eyes fully pigmented. As a matter of fact also the Silky breed, though quite white in plumage when adult, often -perhaps always- has some buff colour in its down. The resemblance is therefore far from being complete.
Another case which suggests a similar interpretation is that of eye-colour in the mouse, for there black eyes result in F1 from crossing certain pink-eyed mice having coloured coats with certain albinos. But there again one of the parents is obviously not albino, and as we now know from Miss Durham's observations, the eye of the coloured parent though ostensibly pink, really contains a small but definite amount of pigment.
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Animals and plants are alike in the fact that their colours, however produced, may be modified by the presence of additional factors. In each case therefore we must conceive of one lowest or hypostatic colour, and of epistatic factors superimposed on this which produce their several effects. In the Sweet Pea the lowest colour is red, which is turned to purple or blue if the factor having this power is present. Similarly in the mouse the lowest colour is chocolate, which becomes black if the black factor is added, and so on. The intensity and also the distribution or pattern of colours behave m descent as if they also were governed by such superimposed factors, though as will presently appear, it is not certain that this mode of representation is strictly correct. However this may be, we are safe in regarding the pigmentation of animals and plants as a character usually resulting from the combined operations of several distinct factors, transmitted separately in heredity.
Applying conceptions which have lately become current in physiology Cuénot suggested that the determiners which modify colour in the mouse, for instance, may be distinct diastases acting on a single chromogen substance. In the present state of physiological chemistry it is, I suppose, impossible to speak with confidence as to the nature of the bodies concerned and we must keep an open mind. Nothing yet precludes the possibility that there may be one diastase responsible for the production of colour, and another set of bodies which, acting in the presence of the diastase and of the chromogen, determine the quality or shade of the colour.
So in the mouse, the wild grey colour results from the joint action of at least three factors : (1) the colour, which, if no epistatic factor is present, would be chocolate ; (2) a black determiner, which causes black pigment to appear ; (3) the agouti-factor, G, which gives the hairs their banded appearance and also causes some yellow pigment to be formed in them. In rats a. black variety exists because the factor G may be absent, but no chocolate variety has been recorded because the factor for blackness has not yet fallen out. If
* As pointed out above, Coénot's suggestion that in the case of mice the agouti-factor, G, is allelomorphic to the factor for blackness, B, is not an adequate representation of the phenomena. (See p : 76.)
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a chocolate rat were to be produced, then by crossing it with the wild grey type, blacks must occur in F2,just as they are known to do in the case of the same mating in mice.
The facts compel the recognition of such a series of determining elements, and it is perhaps simpler to imagine these elements as distinct from the exciting cause, and additional to it, while remembering the possibility that they may in reality be only modifications of it.
Similarly in attempting to express the genetic interrelations of the several patterns of a colour as depending on the existence of definite factors, we have to bear in mind that we are only using a convenient symbolism. It is not incumbent on us to believe that there are any physiological substances which have the power of governing the distribution of the colour. Experiment shows that the power to cause the colour to be uniformly distributed as in the "self" type, or to be restricted to special regions of the body as in the Dutch rabbit, for instance, can be carried by the gametes, and that when these two possibilities are combined in heterozygosis, they segregate in gametogenesis.
This being so, the two possibilities may thus be represented symbolically as two factors, having regard to their effects on the configuration of the resulting zygote ; but if we must attempt to imagine an answer to the question, wherein does the distinction between self pattern and Dutch pattern physiologically consist, we should, I suppose, refer it rather to differences in the distribution of one of the chromogenic factors than to the presence or absence of an additional element. In the self-coloured rabbit the two colour-producing elements are generally distributed over the skin, while in the Dutch rabbit either the chromogen or the diastase -if these be the critical substances- is restricted to certain areas. The colours in the pied animal thus come out in certain patches just as do lithographic colours upon the prepared parts of the stone when the ink is applied to the whole surface.
As the black rabbit or mouse is an animal from which the grey determiner, G, is absent, so the pied animal is one from which the self-coloured distribution is absent. Nevertheless the essential distinction between the two forms must surely be quantitative. In the self-coloured type one of the
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substances, say the chromogen, is distributed over the whole surface, but in the Dutch-marked, for example, it is reduced in quantity. The reduction however occurs in a fairly definite way, leading to the formation of a type having a recognizably distinct pattern. It does not seem an unreasonable speculation to suppose that we have here to deal with a condition in which the amount of the substance is insufficient to cover the whole region which it occupies in the self-coloured type, though why it should be restricted to one special region more than another it is impossible to say.
If the definite pied phases are to be thus regarded as representing quantitative diminution in the development of one of the determining substances, we may make a similar supposition in regard to the diluted colorations already mentioned in the case of mice. In the diluted colours the reduction in quantity, instead of diminishing the coloured area while keeping the intensity of the colour, is effected by diminishing the intensity of the colour while the totality of the distribution is retained. The black Dutch-marked mouse may thus be imagined to be a mouse in which one of the colour-factors exists in its full intensity, though there is not enough of it to cover the skin, while in the blue mouse the factor is generally distributed over the skin but in a dilute condition. In both cases alike the subtraction-stage as we may call it is a fairly definite stage in the reduction of the amount of pigment.
A physical analogy-doubtless imperfect, but nevertheless instructive-may be drawn from the way in which various oils distribute themselves over the surface of a liquid with which they do not mix, some forming large circumscribed patches of greater thickness, which may be compared with the patches on the Dutch rabbit, others spreading in a thin layer over the whole surface, like the dilute colours spread over the whole. The analogy breaks down at the fact that in the oils the physical distinctions to which the different behaviours are due cannot be transferred from one oil to the other, whereas in the rabbit this is accomplished-a fact which entitles us to represent the several properties as distinct and transferable factors. Thus the results of the cross between a black-and-white Dutch- [Dutch-marked]
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marked mouse and a self-coloured blue may be represented as due to the re-combinations of the two pairs of factors.
[Table not reproduced in this version]
The recognition of these subtraction-stages becomes important when we attempt to estimate the minimum number of factors necessary to produce the results we perceive. Symbolically the various subtraction-stages may be represented as depending on the removal of distinct factors, but physiologically they may be caused by special quantitative subtractions from one of the factors causing the production of colour, and consequently an economy of hypothesis may be made.
In exceptional cases the pattern which an albino form is carrying, if the expression be permitted, can be actually recognized by inspection though no real colour is developed. For example Lock noticed the following case in edible peas (Pisum sativum).Certain varieties of peas have brown anastomosing lines on the coats of the seeds. These peas are called maples in England (poisperdrix of French seeds-men). The maple skin occurs only in the seeds of strains which have coloured flowers. Such plants crossed with an ordinary white-flowered type having a plain seed-coat gave this result :
[Table not reproduced in this version]
Among the white-flowered group in F2 were some plants which bore seeds showing traces of the maple marking, not as brown lines, for no actual pigment seemed to exist, but as a damasked pattern showing where the mapling would have
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been if the plant had been a coloured one. Lock (176)has spoken of this faint pattern as the "ghost" of the mapling.
Mudge (204) has observed a very similar phenomenon in young albino rats. When the hair is short the coat may be seen to be similarly damasked, those parts which would be pigmented if the animal had pigment looking different in consistency from the rest.
What the exact difference between the hairs in these areas and the rest may be has not been ascertained, but evidently it must be a modification due to the existence of one of the factors for colour in those hairs. They are the parts prepared to develop colour if the other element were present in them. In black leopards and black kittens a similar damask effect can often be seen, the parts which in the spotted leopard or tabby cat would be light being distinguishable on careful examination. As it is not yet known whether black is dominant or recessive in these cases the exact meaning of these marks is uncertain.
Both in animals and plants there is satisfactory proof that whiteness, the absence of colour, may be due to partial or complete suppression of the pigment-factors and not merely, as in the albino, to their absence. This suppression as caused by a dominant, epistatic factor. White individuals containing such a factor are more or less totally dominant whites,whereas whites due to the absence of one or more pigmentation-factors are recessive whiles.A cross between dominant and a recessive white may obviously cause coloured individuals to appear in F2, if the factors for pigmentation were introduced by the parental types ; for in F2,there will be individuals lacking the suppressing factor.
Confusion has been introduced into these analyses by the use of the term "latency" in application to those factors which cannot be perceived without breeding tests. This difficulty has occurred especially in regard to albinos, though it pervades the whole system of factorial analysis. Albinos, for instance, in any species may have the most diverse factorial composition. All that is common to them is the absence of colour, i.e.,if we adopt Cuénot's suggestion, of the chromogenic substance. The composition of each albino may be ascertained by crossing it with a coloured type and raising the F2 generation. If the coloured type chosen be
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in all its characters or recessive, to use the simpler if less accurate term-to all the factors of the albino, the composition of the albino may be seen even from F1. For instance, when bred to a pure black, a GG albino will give greys only ; a GB albino will give equal numbers of greys and blacks ; while a BB albino will give blacks only. Conversely it is possible to manufacture by suitable matings albinos of each composition. For example, albinos extracted from chocolates can only bear the chocolate determiner. Those from black mice must all bear the black determiner if the families have been large and no chocolate has occurred. So, those from greys must all bear the determiner G, if in sufficient numbers no blacks or chocolates have been produced. With regard to pattern and saturation or dilution of colour the case is exactly the same. An albino from Dutch-marked parents cannot bear the self-colour factor, and one from blues cannot bear the saturation-factor.
It is this fact, that in most cases the albino will be bearing the determiner proper to the colour of the last coloured parent from which it was extracted, which has led several writers to speak of these colours or patterns as "latent" in the albino. This mode of expression is much to be regretted. There is no "latency" of black, or grey, or self-colour, as a whole in the albino. Certain factors which are essential to the production of those features may be present in any albino of unknown origin, but this fact does not in any way touch the question of the purity of the germ-cells, as has been quite erroneously suggested. Sulphate of copper is blue and chloride of copper is green, but it would be incorrect to speak of blue as latent in sulphuric acid, or of green as latent in hydrochloric acid ; nor has the acid obtained from chloride of copper more of "greenness" in it than has the same acid obtained from sodium chloride.
Taking the evidence respecting the genetics of colour as a whole, though much remains which is obscure, as has been stated, especially in the discussion of yellow in the Rodents, there can be no reasonable doubt that with rare exceptions it will be found possible to express the whole series of phenomena as due to the combination and recombination of a limited number of recognizable factors which are treated by the cell-divisions of gametogenesis as units.
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As the nature and properties of each of these units are successively determined, we cannot doubt that additions will be made to the number of examples, already not inconsiderable, in which a fixed interrelation can be proved to exist between the units which govern colour and those responsible for form and other physiological attributes.
In any attempt to picture the process of Evolution the group of genetic. phenomena discovered in regard to colour has extreme value and interest. We thus are at once provided with clear illustrations which enable us to see the nature, if not as yet the causation, of Variation, and the significance of those particular Variations which we call reversionary. Such illustrations may well serve as testcases, by which the truth of evolutionary systems may be gauged. Though the result of these trials may largely prove destructive, the facts are not without a constructive bearing. One positive deduction cannot be overlooked : that the organism is so built up that definite additions to, or subtractions from its totality may readily be made by Variation, and that the consequence of such alteration of the ingredients may be recognizably definite, or to use another term, specific.
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