Problems of Colour in Nature

Before I begin this paper, I wish to make two apologies to the Society – first for not having let the secretary know the subject on which I intend to write, and secondly for choosing such a vague title. But I will try to make amends by stating at the outset the questions which I shall place before you for discussion. The phenomenon of protective colouring in nature cover too wide a field for the scope of a paper of this kind, so I shall confine myself to three smaller questions which, though not so widely interesting, nevertheless will serve as pabulum for thought & discussion.

The questions are as follows :-

1. Is Albinism common in dark coloured species?
2.  What regulates variations in colour?
3. Is there any correlation between colour and constitution?

Albinism, which as everyone knows, is due to the absence of those tiny particles of colouring matter which are responsible for the colour of the skin, was first observed among the negroes of West Africa by the Portuguese who called them Albinoes. In the human race, with very rare exceptions, this albinism is complete, that is to say, it affects the whole body and continues from the moment of birth till the hour of death. All the peculiar characteristics of the albinoes, the white silk-like hair, the pink eyes, and the rough texture and dull milky appearance of the skin have caused them to be regarded as freaks and consequently it has been often asserted that they are deficient also in mental vigour. But this is by no means the case and indeed the only two albinoes of my acquaintance are men of exceptional intellectual ability. But to pass to the general question as to the c0lour of the species in which albinism is most common. 

Reference has already been made to the occurence of albinism among negroes. It has frequently been observed among Indians also, and it would be interesting to have statistics as to the complexions of the members of those families in which albinoes appear among white races. I have not been successful in providing myself with any such set of statistics, bt in the case of one of the two men referred to above, all his relations whom I have met have dark complexions and black hair. Of the other case, I have no knowledge. When we consider albinism among the lower animals we find abundant evidence to support the theory that it occurs more commonly among dark species. White blackbirds, white starlings, & white crows have frequently been met with and cases of partial albinism are very numerous. In the spring of this year in Monsal Dale, I had the good fortune to observe a ring-ousel which was almost completely white and last year I saw traces of albinism in one of the sparrows which breed on the road to the racecourse. Among rabbits there is an albino greed – the Himalayan, I believe is the name by which it is generally called – and, though exhibiting complete albinism at first, in a few weeks their ears, feet and tail become black.

 

Now this particular breed was produced by crossing two kinds of silver-grey rabbits and is curious to find that these silver-greys are at first black and gradually change their colour. Here the question at once arises, does the dark colour of the silver-grey in early life represent the original colour of the species? If we answer in the affirmative, then the change of the Himalayan breed from pure white to black in certain parts must be explained as a reversion to the colour of the original species. This is the view which Darwin accepted. But if there are any laws regulating such changes of colour why cannot the change from black to silver-grey be explained as a reversion to the original colour? This is a difficulty which I would suggest for discussion.

Apart from the cases discussed above, ithas been observed that among fishes, albinoes though extremely rare, as a rule, are by no means uncommon in the darker species.of course itis a well known fact that deep sea fishes are almost invariably of a dark colour, and they tend to become lighter when removed from their natural haunts. This change has been explained as due to the contraction of the pigment cells as chromatophous which are very sensitive to external irritation, and are rapidly affected by the change from darkness to light. But the change must not be confused with genuine cases of albinism in which there is a total absence of these pigment cells.

The influence of domestication on certain species is to cause a tendency to albinism; e.g. in the chinese carp the dark chromatophous are changed to cells with a yellow pigment which gives them their golden appearance. While searching for material for this paper, I have found the colour yellow occurring several times in connection with the phenomena of albinism. In addition to this instance of the goldfish, Himalayan rabbits are sometimes of a yellow colour, and albinism is by no means uncommon among canaries. I shall be glad if any members of the society can explain the nature of the connection, if such a connection exists.

I must now pass on to the consideration of the second question. What regulates variation in colour? and lest I should mislead you, I must explain that I mean variations in colour in one and the same species. A thorough discussion of this question will necessarily bring in the question of protective mimicry and aggressive colouring, but it is not on that aspect that I intend to dwell.

A comparison between the colours of domesticated animals and the colours of animal in the wild state at once reveals a much greater amount of variation in the case of the former. Rabbits, pigeons, fowls, horses, and indeed most domestic animals show variations which seem to defy any attempt to reduce them to any regular law of variation and even among creatures in the wild state, the modification in colour are often very striking. The natural explanation of such variations at once suggests itself as a change of environment, and without doubt it is in this that we must seek for the answer to our question. It has been clearly proved that birds of the same species show variations not merely in colour but in other characteristics according to the locality in which they are found and these differences have sometimes been directly attributed to action of temperature, e.g. in the case of the birds of America which show marked differences as one proceeded from North  to South, and also I believe in the case of certain European butterflies. But surely it would seem to be impossible to separate temperature from all the other conditions of  life & environment, and prove definitely that variations are due to that alone.

The nature of the food, one would think, must be of considerable importance. Everyone knows that black bullfinches can be produced by feeding them on hempseed, and although Darwin, arguing from the fact that we see considerable variations in colour among many domestic animals, the nature of whose food varies but slightly, comes to the conclusion that the nature of the food has very little influence on colour, we can hardly dismiss it altogether from the many conditions of life whose sum total is contained in the term, environment.

But apart from environment, we have to consider the influence of heredity. We have seen that in animals in the wild state, the variations are but slight and we may say that usually the characters of the parent are reproduced with remarkable persistence in the offspring from generation to generation. But in domestic animals the tendency to vary is much more marked and is evidently due partly to the artificial conditions of life and partly to the crossing of different species. Of course. it is only natural that if two distinct species are crossed, the characters of both will reappear in the offspring to some extent and often several generations of promiscuous breeding we shall not be surprised to find considerable variations in colour.

It is now time to hurry to the last question. Is there any correlation between colour and constitution? It is a matter of wide belief that certain constitutions peculiarities are usually to be found in connection with certain complexions, both in man and in the lower animals. Some colours are more attractive than others and hence we find that the colours of animals and plants often determine their susceptibility to external influences. Instances are on record of an albino negro being subject to the attacks of insects while his companions escaped entirely. It is well known that white chickens suffer more from the gapes, which is caused by the presence of a parasite in the trachea; and that white verbenas are more subject  to attack of blight than other coloured varieties.

These instances merely prove that colour plays an important part in determining constitution in so far as it is modified by external influences. Bur Darwin gives a striking piece of evidence which seems to prove that apart from these external influences there is an inherent difference between the constitutions of differently-coloured animals of the same species. The effect of a certain plant on white pigs in Virginia is to colour their bones pink and to cause their hoofs to drop off, whereas black pigs can eat the same plant without suffering any harm. There we have the same external influences working upon members of the same species with different results in the case of black and white members. Among dogs, too, it has been noticed that white terries invariably suffer more from distemper than terries of other colour, Surely these examples provides strong evidence that there is some direct connection between colour and constitution.

I an sorry if owing to the shortness of the time which I have been able to devote to treat a subject of such interest in too superficial a manner. But if I succeed in merely suggesting one or two questions which will provoke discussion, I shall not have written it in vain.