acquired; but some animals do not acquire even simple habits, although they would apparently prove of great advantage. Sir Herbert Maxwell tells us of ducks that feed only on the surface of the water. They have not acquired the habit of diving to reach their food, although they may be seen feeding on the floating débris brought to the surface by closely allied breeds that seek their food under water.

CHAPTER XI.

EXTERNAL INFLUENCES.

THE three principal factors in producing an organism life-force, matter, and environment-vary in their respective potencies in different races, and among individuals of the same race under different conditions.

The prepotency of Specific life-force-the force that moulds the type-is greater the more complex the organisation, while, as we descend in specialisation, the influence of one or of both the other factors becomes more evident; that is, lower organisms are more affected by external conditions, food, climate, &c., and display greater differences in expression of type, than those more highly specialised.

The differences in the respective potencies are readily observable in plants; their size, flowers, and fruit may be greatly varied by more or less favourable conditions of growth. If heat is inadequate, the plant (if it live)

will be stunted; if kept in the dark, its colour will be affected; and insufficiency of moisture will be evident in the foliage. A plant grown in a valley may change greatly in appearance on a mountain; and some trees when transported to a colder climate, although they live, may fail to blossom or to mature seed.

Man can, however, do comparatively little to alter the form, size, or appearance of seed -the part of the plant that may be taken to represent the life-force. When the fruit that envelops some seed is greatly developed by high cultivation, both the quantity and fertility of the seed are usually greatly diminished, but the appearance of the seed itself is only slightly modified, if at all. On the other hand, the vitality of seed, matured under even highly unfavourable conditions, does not seem to be seriously impaired, for the young plants from such seed readily respond to generous treatment, and may at maturity approach the normal size. It is also remarkable that reproduction is stimulated by unfavourable conditions of life: trees growing under adverse conditions yield the largest quantity of seed, and animals in like circumstances are most fertile. Statistics show that the population of Ire

land never increased so rapidly as at the time of the great famine, and similar experience attends the periodical famines in India. By the same natural law the birth-rate in London is much higher in the poorest parishes than in the wealthiest.

The last efforts of life seem to be devoted to the perpetuation of the race rather than to the preservation of the individual life. From this we may infer Nature's reluctance to lose a type.

These phenomena, whether they arise from external or internal conditions, give no indication of a tendency towards specific variation.

Environment.

Darwin calls attention to the correlation between the colour of animals and that of the country they inhabit, and to the changes in the colour of some animals during certain seasons. This correlation he attributes to natural selection and the struggle for exist

ence.

There is, undoubtedly, frequently a correspondence between the colour of animals and that of their habitat, and some insects exactly resemble the leaves or twigs of the

trees where they are to be found. But the exceptions to this correlation are numerous enough to show that correlation is not due to environment.

The blackcock differs much in colour from his grey mate; the mountain-hare, bluish grey in summer, becomes white in winter, but the fox, living on the same mountain, retains his colour unchanged. Grouse and ptarmigan live in almost the same localities, and in summer are similar in colour; but as winter approaches the grouse becomes a darker brown, the ptarmigan white, and the change begins long before the advent of snow.

Some butterflies are attractively and brilliantly coloured, whilst others in the same locality can hardly be distinguished from the leaves and twigs on which they rest.

Most animals in the arctic regions are white, but the sable and the musk-sheep are conspicuously brown, and other animals are of different hues.

Mr Alexander Wallace, in explanation of these inconsistencies, says: "Whenever we find arctic animals which, from whatever cause, do not require protection by their white colour, then neither the snow nor the glare have any effect upon their colouration."

But this explanation seems really an argu