If the reader will kindly refer to the tables of variations in the bats' wings (Figs. 14-17), he will see that there are a great number of bones which vary in length and vary independently. And if he will also refer to Fig. 18, in which seven species of bats are compared, he will see that the differences arise from the increased length of one set of bones in one species and another set of bones in another species. Now, let us suppose that the long, swallow-like wing of the noctule, a high flyer with rapid wing-strokes, that catches insects in full flight, and the broad wings of the horse-shoe, a low flyer, flapping slowly, and, at any rate, sometimes catching insects on the ground, and covering them with its wings as with a net; let us suppose, I say, that to each species its special form of wing is an advantage. Among thousands of independent variations in the lengths of the bones there would be occasional combinations of variations, giving either increased length or increased breadth to the wing. In the noctule, the former would tend to be selected; in the horse-shoe, the latter. Thus the wing of the noctule would be lengthened, and that of the horse-shoe broadened, through the selection of fortuitous combinations of variations which chanced to be favourable. Now, each individual bone-variation is, we believe, due to some special cause; but the fortunate combination is fortuitous, due to what we term "mere chance." Darwin believed that chance, in this sense, played a very important part in the origin of those favourable variations for which, as he said, natural selection is constantly and unceasingly on the watch. And there can be little question that Darwin was right. We must now consider very briefly some of the proximate causes of variations. In most of these cases we cannot hope to unravel the nexus of causation. When a plexus of environing circumstances acts upon a highly organized living animal, the most we can do in the present state of knowledge is to note--we cannot hope to explain--the effects produced. All readers of Darwin's works know well how insistent he was that the nature of the organism is more important than the nature of the environing conditions. "The organization or constitution of the being which is acted on," he says,[EF] "is generally a much more important element than the nature of the changed conditions in determining the nature of the variation." And, again,[EG] "We are thus driven to conclude that in most cases the conditions of life play a subordinate part in causing any particular modification; like that which a spark plays when a mass of combustible matter bursts into flame--the nature of the flame depending on the combustible matter, and not on the spark."