36

VARIATION AND INHERITANCE.

mitted, be looked upon in great part as a subsequently acquired character. The laws of variation are therefore those which should naturally be considered first; and from a consideration of these it is thought a clue at least to the nature of hereditary transmission may be obtained.

MR. DARWIN has carefully pointed out that variation may be either definite or indefinite: that is, it may result directly from the definite action of some one or more conditions; or it may be the result of a slight change in numerous complex relations, so that it is apparently irregular or indefinite.

Variations in living beings may be fitly compared with variations in the weather. Thus when the temperature falls below a certain point a fall of snow may be confidently predicted instead of a fall of rain. The snow is known to result from a low temperature. It cannot, however, be predicted that a shower of rain will occur at a certain time and place in cloudy weather, because, although no one doubts that the occurrence of a shower is the result of certain physical conditions, these are so numerous and complex that rain cannot be predicted from several of them having been observed; hence showers are commonly looked upon as the result of accident or chance.

Thus definite variations occur constantly in some

38

ARE VARIATIONS ADAPTIVE?

animals and plants under the influence of certain conditions. For instance, Mr. Gould believes that birds of the same species are more brightly coloured when living under a clear atmosphere than when living near the coast or on islands. Wollaston is convinced that residence near the sea affects the colours of insects. Many maritime plants belonging to very different families are glaucous; and others are succulent. A dry locality undoubtedly produces a thorny condition in some shrubs; and pubescence is a common result under similar conditions. The same species are often perfectly smooth in wet meadows, and covered with down in dry places.

A question of great importance arises with regard to definite variations, Are they in themselves adaptive? Is the bright colour of birds living under a bright atmosphere at the same time the result of the strong light and beneficial to the creatures which live under it? Is the hairiness of plants in dry places the result of the dryness and beneficial to plants as a means enabling them to withstand drought ?

This question must be answered with great caution, since it is one of extreme difficulty. We do not know why hairiness appears in a plant in a dry place; yet it is doubtless connected with the nutritive function. But whether the hairs

NUTRITIVE VARIATIONS.

39

become of use to the plant in connexion with its growth in a dry place, or not, is probably determined by natural selection: even if the hairs are needful to the plant to enable it to withstand drought, we cannot tell that they have not gradually arisen by the survival during many thousand generations of those plants only which tend to become hairy in dry places, at least amongst those species which are frequently subjected to conditions of drought. The same may be said of most definite variations arising from external causes. For instance, certain birds and mammals become white in winter, perhaps from the direct effect of cold on certain peculiar constitutions; yet if this be so, natural selection is probably the main instrument by which the effect has been produced, those animals surviving continually which tend to become white in winter and to resume their coloured coats in the ensuing summer; for it will be readily conceded that assimilation to the colour of surrounding objects is an important means of concealment.

There are, however, another set of changes which arise from the direct effect of internal conditions connected with certain laws of nutrition, which are undoubtedly adaptive in their nature. These formed the basis of Lamarck's hypothesis, and have probably been great factors in the origin of the forms we see around us, although of late their importance has

40

INDEFINITE VARIATIONS.

been, to a great extent, lost sight of. Such are:the tendency of tissues to become developed by use; the increase in the size of bones, and of intermuscular septa and ridges of bone, with increased development of muscle; the hardening of the skin from the action of external agencies, by hypertrophy, to enable it to resist external influences. Such are the induration of the blacksmith's hand by labour, and the broadening of the palm to produce a more effective grasp.

Indefinite variation chiefly differs from that which is definite in not affecting numerous individuals in the same locality, and in its much slighter degree. Individual differences are of this kind. It is indubitable that the occurrence of grey, blue, or dark eyes, or dark or light hair in an individual depends on conditions; yet no one can say on what special conditions. As we have already seen, individual differences are sometimes as considerable as definite variations; this is especially seen in polymorphic genera.

With regard to the question, are indefinite variations adaptive? the same answer must be given as in the case of definite variations, as one merges into the other by numerous transitions: it is probable that many individual differences connected with the nutritive function are the result of use and disuse, and are therefore adaptive. This mate