There are even characters the biological use of which is unknown to us, but in regard to which we can affirm that they have a use. Thus Eigenmann described the larva of an American eel, which differs from other so-called 'Leptocephali' in that a row of seven black spots runs along its side. Apparently all these lie upon the side turned towards us, but in reality they are distributed on both sides, three lying on the left and four on the right, and so arranged that they look like a single row of spots at regular intervals, for the flat little fish is absolutely transparent. The habits of this larva are not yet known, but we may conclude that this appearance of a simple row of spots must have some value for the animal, for such a significant asymmetry could not have arisen for purely internal reasons (Fig. 107, C). It is possible that the fish is thus made to resemble parts of some marine alga, and that it is thereby protected from many enemies; that there is not a complete row upon each side may depend upon the fact that the two rows would be visible at the same time, and that they would blur each other in the eyes of the swimming enemy, and so destroy the resemblance of the picture to its unknown model.

But it cannot be denied that there are characters which have no special biological significance. There are doubtless many such characters, which stand beyond the threshold of good or bad, and which are therefore not affected by personal selection; it is difficult and often impossible to point these out with certainty. The shape of the human nose and of the human ear, the colour of the hair and of the iris, may be such indifferent characters whose peculiarities are to be referred solely to germinal selection. On the other hand, I would not venture to assert that the gay colouring and the complex markings on the wings of our modern Lepidoptera are always and in all cases unimportant, even when we cannot interpret their details either as protective, or as a sign of nauseousness, or as mimetic. The usually very exact similarity of the colour pattern in the individuals of each species seems to point to the intervention of personal selection in some form or other, for in what other way could such a large majority of variations in the same direction have developed in the germ-plasm as this constancy of the character indicates.

We know, of course, that the colours of butterflies and moths can be caused to vary through external and especially climatic influences, but this would only account for simple modifications of colour, and not for the origin of the complex colour patterns that actually occur. I therefore believe with Darwin that sexual selection has had much to do with this by giving a slight preference to the variations produced by spontaneous germinal selection, and thus preventing the majority of varied ids once acquired from being scattered again, but always collecting more of them, and so securing free play for the increase of the new character through intra-germinal processes. In this way have arisen not only the brilliance of our Lycænidæ and of the large Morphidæ of South America, but also many of the coloured spots, streaks, bands, eyes, and other components which have gradually in the course of time evolved into the complex colour pattern of many of our modern butterflies. I should like to remind any one who doubts this of a fact which corroborates the view that personal selection has co-operated in the production of these colours—I refer to the inconspicuous colouring of the females of many of these brilliant males—while in contradistinction to these cases there are other species in which both sexes are alike brilliant, so that it is impossible that mere spontaneous germinal selection can have determined that the females, because of their femaleness, should vary in a different manner from the males.

But while I believe that sexual selection in particular has had much to do with producing the colours of Lepidoptera, the basis of all these colour variations must still be looked for in germinal selection, and we shall see later on how it is possible to think of the diversified and often relatively abrupt transformations of marking as the resultant of the co-operation of climatic influences with germinal selection.

Of course there must also be unimportant changes in butterfly-markings which depend solely on the internal play of forces in the determinant system, and to this must be referred the markings of many of the 'variable' species whose variations are mere fluctuations in the details of marking, which have therefore caused much trouble to the systematists. Truly unimportant variations will rarely or never combine into a 'constant' form, and the fact that there are species which are 'variable' in such a high degree is enough to make us refer their variations to their lack of importance, for if they possessed any biological value the less valuable among them would gradually be removed by selection. Perhaps the variable species of certain moths like Arctia caja, and especially Arctia plantaginis, the little 'bear' of the Alps and Apennines, must be reckoned among these. But from the fact that there are such fluctuations in the markings of Lepidoptera, it seems to me that we must conclude that species which show a high degree of constancy in their markings have been influenced by selection, or by climatic influences which turned the play of forces within the determinant system in the same direction in all individuals. All these considerations and conclusions are quite sound and serviceable theoretically, but they are difficult to apply to individual cases, and where this is attempted it must be with the greatest caution, and, if possible, on a basis of investigations specially undertaken for the purpose; for how should we know whether a species which to-day is highly variable may not a geological epoch later become a very constant one? We must in any case assume that marked fluctuations of characters are associated with many transformations.