The great diversity of character and capacity among men doubtless arises from their greater and more complex needs, relations, and aspirations. The animals' needs in comparison are few, their relations simple, and their aspirations _nil_. One cannot see what could give rise to the individual types and exceptional endowments that are often claimed for them. The law of variation, as I have said, would give rise to differences, but not to a sudden reversal of race habits, or to animal geniuses. The law of variation is everywhere operative--less so now, no doubt, than in the earlier history of organic life on the globe. Yet Nature is still experimenting in her blind way, and hits upon many curious differences and departures. But I suppose if the race of man were exterminated, man would never arise again. I doubt if the law of evolution could ever again produce him, or any other species of animal. This principle of variation was no doubt much more active back in geologic time, during the early history of animal life upon the globe, than it is in this late age. And for the reason that animal life was less adapted to its environment than it is now, the struggle for life was sharper. Perfect adaptation of any form of life to the conditions surrounding it seems to check variability. Animal and plant life seem to vary more in this country than in England because the conditions of life are harder. The extremes of heat and cold, of wet and dry, are much greater. It has been found that the eggs of the English sparrow vary in form and color more in the United States than in Great Britain. Certain American shells are said to be more variable than the English. Among our own birds it has been found that the "migratory species evince a greater amount of individual variation than do non-migrating species" because they are subject to more varying conditions of food and climate. I think we may say, then, if there were no struggle for life, if uniformity of temperature and means of subsistence everywhere prevailed, there would be little or no variation and no new species would arise. The causes of variation seem to be the inequality and imperfection of things; the pressure of life is unequally distributed, and this is one of Nature's ways that accounts for much that we see about us. VIII WHAT DO ANIMALS KNOW? After the discussion carried on in the foregoing chapters touching the general subject of animal life and instinct, we are prepared, I think, to ask with more confidence, What do animals know? The animals unite such ignorance with such apparent knowledge, such stupidity with such cleverness, that in our estimate of them we are likely to rate their wit either too high or too low. With them, knowledge does not fade into ignorance, as it does in man; the contrast is like that between night and day, with no twilight between. So keen one moment, so blind the next!