THE FORMS AND COLOURS OF LIVING CREATURES.
In the Essay on Animals and Plants, which appeared in the September Number of this Review, the names were given of the principal groups in which the prodigious multitude of living creatures (existing or known to have existed) have been classified by naturalists. It was therein also indicated that these various groups, and all the subdivisions of such groups, are distinguished one from another by variations in the forms and structures of the creatures which compose them. This fact alone would prove that very many differences in form must exist; but, indeed, a very slight knowledge and a very cursory examination of animals and plants would suffice to show this even to any one who knew nothing of the scope or nature of biological classification. In truth, to the non-scientific observer who feels an interest in living things, the difficulty may seem to be rather how to find general resemblances than how to detect differences between creatures which seem so totally diverse as do humming birds from whales, bees from buffaloes, or the numerous African herds of antelopes from the grasses on which they feed.
Nevertheless it was pointed out in the second Essay of this series[56] that all living creatures do agree to a certain extent in the form and structure of their bodies, inasmuch as their bodies are always bounded by curved lines and surfaces, while, if we divide the body of any animal or plant its structure may always be seen to be heterogeneous—that is to say, composed of different substances, even the simplest showing a variety of minute particles (granules) variously distributed throughout its interior. It has also been pointed out[57] that all living creatures agree in beginning life in the form of a small rounded mass of protoplasm. But all animals and plants further agree in that each kind has its own proper size, shape, structure, and colour, and each (as we shall hereafter see) shows a positive unity in its fundamental constitution, co-existing with the heterogeneity above referred to.
But though each kind has its own proper size, shape, structure, and colour, yet these vary more or less in different individuals, and the degrees of variability are different in different kinds both of animals and plants.
As to size, although most living creatures have certain limits which they rarely exceed or fall below, yet many organisms vary greatly in this respect. Thus, that familiar weed, the common centaury (Erythræa centaurium), may vary in height—according to the soil and other external conditions—from half an inch to five feet.
As to figure and structure there is more constancy, and the amount of variation which may in these respects be found between different individuals of the same animal species, is generally but slight. In plants and in plant-like animals much greater differences exist as to external configuration; but even in them the internal structure of each species varies but little.
Colour is a character which some readers may be disposed to regard as extremely inconstant. We are familiar with many differently coloured varieties of our cultivated flowers; and white blackbirds, and black leopards are not very uncommon objects. Nevertheless, colour is really a character of much constancy, and is one not only constantly present in different individuals of one kind of plant or animal, but is one constantly present in particular groups of kinds.
Thus, for example, all the English plants of the dandelion order which have opposite leaves, have yellow flowers, with the single exception of the eupatory (Eupatorium cannabinum), and whole groups of butterflies are respectively characterized as being blue, or white, or yellow.
We have seen that the life of every living being is accompanied by, and may be described as, a series of adjustments of action and structure to external conditions which surround it. Accordingly we may expect to find that the sizes, shapes, structures, and colours of living beings bear relations, which are in very many cases obvious, to their external circumstances, as directly favouring their nutrition, reproduction, or preservation from external injury.
Every living creature must be either fixed (like a rooted tree), or capable of spontaneously moving, or of being passively drifted from place to place, and must have a structure and figure suitable to one or other of these conditions.