Many variations which frequently occur among both wild and domestic animals do not persist in nature.
As instances of such variations we may mention pure albino forms, that is to say those in which pigment does not occur in the eyes.
It is easy to see why this variation is not allowed to persist in nature. Its possessors are handicapped by bad eyesight, and so have no chance of surviving in the struggle for existence. It is thus that natural selection acts. On the other hand, white species with pigmented eyes are fairly numerous. These enjoy normal eyesight, but labour under the disadvantage of being easily seen by their foes. Hence we find that white species generally either occur in a snowy habitat, or are powerful and both able and ready to defend themselves. In this connection it is interesting to notice that in New Zealand all birds, whether introduced or indigenous, are particularly liable to albinism. Owing to the fewness of their enemies these albinistic forms are able to persist.
A variation, or rather a mutation, that frequently occurs among domesticated birds, but which is seen in very few wild species, is that which takes the form of white primary feathers on the wing. This variation must often occur in nature, but it rarely establishes itself, apparently because white feathers do not resist wear so well as coloured ones do.
Black-and-yellow colouration occurs in several widely separated species of birds. The arrangement of the two colours follows to some extent the same rules as the black-and-white combination.
Several birds have a yellow body with black head, wings, and tail, such as—
The Black-headed Oriole (_Oriolus melanocephalus_).
The Black-and-Yellow Grosbeaks (_Pycnorhamphus icteroides_, _P. affinis_) (cock).
The Common Troupial (_Icterus vulgaris_).
In others the black on the head is nearly or quite suppressed, that on the tail remaining to a greater or less extent; such are—
The Golden Orioles (_Oriolus galbula_, _O. kundoo_, etc.).
Several fly-catchers of the genus _Piezorhynchus_ (males only).
[Illustration: INDIAN BLACK-HEADED ORIOLE]
We have said sufficient to show that certain combinations of colours recur in nature in species which are neither nearly related to one another nor subjected to similar environment. For such phenomena it is difficult, if not impossible, to account on the theory that natural selection, acting on minute variations, is responsible for all the varied colouring of the animal kingdom. The facts, however, are in accordance with the supposition that the organism is the result of the growth and development of a number of units or biological molecules which exist in the fertilised egg.