The science of life : fully illustrated in tone and line and including many diagrams
BOOK 3
blue fox skins of commerce. The more southerly red fox also has, as a comparatively rare variety, the silver fox, with black, silver-tipped hair, and the less uncommon cross fox with a cross mark on the shoulders. Members of the different types may be found in a single litter. The same holds for the white and grey varieties of some kinds of herons.
Many plants habitually exist in two or
Orbit of
es
pe Upper eat Tunnel for Nerve to , ooth skin of muzzle
Orbit of Eye
Upper Canine Tooth
/ Tunnel for nerve to skin of muzzle
Fig. 140.
Both are drawn from the left side and to the same scale.
of the fancy breed is shortened.
three such varieties. The common milkwort (Polygalum) may have white or red or bright blue flowers, and the comfrey (Symphytum) is not in the least particular whether it shall be purple or white.
Besides such sharply marked varieties, between which no blends occur, there may be gradual variation leading insensibly from one extreme type to another. The common skua or jaeger may be wholly
230
THE SCIENCE OF LIFE
~ Pivot of Lower
The skull of a King Charles’ Spaniel (below), contrasted with that of a primitive, wolf-like pariah dog (above).
One or two points are labelled correspondingly in the two skulls to bring out more clearly how the nose
CHAPTER 4
white below, or wholly dark ; and between these two types all intermediates occur, through dark-waistcoated to broad-collared birds and so to narrow collars and finally to no collar at all.
Examples of variation in wild animals and plants might be multiplied ad infinitum. And although because of their conspicuousness we have confined ourselves here to colour-varieties, equally marked variations are found in other points in shape and size, in the internal organs, even in such invisible physiological characters as disease-resistance or longevity. All kinds of living things vary ; the differences are merely a matter of more or less, some varying more strikingly than others.
§ 2 x What 1s a Species ? Pivot of Lower
In the foregoing chapters Jaw
we have made frequent use of the accepted classification of living things; it is a chart that shows us our way about the Animal and Vegetable Kingdoms and the intermediate zone between them. But we have said nothing about the way in which that chart has been drawn. The classification of living things is a laborious task ; it involves the collection of great numbers of specimens from Jaw all parts of the world, and their careful, indexed accumulation in central museums where they can be examined and compared ; it involves the full-time labour of collectors and of museum experts in the various groups. It will be worth our while to glance for a moment at the work of these systematists, as they are called, because the way in which living things resemble and differ from each other is in itself a strong piece of evidence in favour of Evolution.
Naturally we cannot examine the drawingup of the whole map; we shall have to confine our attention to a corner of it. We shall speak only of animals in what follows,