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But even in regard to this case we are reasoning in a circle, not giving "proofs," and no one who does not wish to believe in the selection-value of the initial stages can be forced to do so. Among the many pieces of presumptive evidence a particularly weighty one seems to me to be _the smallness of the steps of progress_ which we can observe in certain cases, as for instance in leaf-imitation among butterflies, and in mimicry generally. The resemblance to a leaf, for instance of a particular Kallima, seems to us so close as to be deceptive, and yet we find in another individual, or it may be in many others, a spot added which increases the resemblance, and which could not have become fixed unless the increased deceptiveness so produced had frequently led to the overlooking of its much persecuted possessor. But if we take the selection-value of the initial stages for granted, we are confronted with the further question which I myself formulated many years ago: How does it happen _that the necessary beginnings of a useful variation are always present_? How could insects which live upon or among green leaves become all green, while those that live on bark become brown? How have the desert animals become yellow and the Arctic animals white? Why were the necessary variations always present? How could the green locust lay brown eggs, or the privet caterpillar develop white and lilac-coloured lines on its green skin?

Sometimes, however, geology does not, on the face of it, come into the reckoning. Thus I might have asked the reader to assist at the digging out of a cave, say, one of the famous caves at Mentone, on the Italian Riviera, just beyond the south-eastern corner of France. These caves were inhabited by man during an immense stretch of time, and, as you dig down, you light upon one layer after another of his leavings. But note in such a case as this how easily you may be baffled by some one having upset the heap of clothes, or, in a word, by rearrangement. Thus the man whose leavings ought to form the layer half-way up may have seen fit to dig a deep hole in the cave-floor in order to bury a deceased friend, and with him, let us suppose, to bury also an assortment of articles likely to be useful in the life beyond the grave. Consequently an implement of one age will be found lying cheek by jowl with the implement of a much earlier age, or even, it may be, some feet below it. Thereupon the pre-historian must fall back on the general run, or type, in assigning the different implements each to its own stratum. Luckily, in the old days fashions tended to be rigid; so that for the pre-historian two flints with slightly different chipping may stand for separate ages of culture as clearly as do a Greek vase and a German beer-mug for the student of more recent times.

Properly, however, this work should be done by the assayer, if only for the following reason. It is assumed that the dry ore of the sampler and of the assayer are the same thing; according to the nature of the ore, this may or may not be the case. The assayer, however, uses the sample which he has dried for his moisture-determination, as the dry ore on which he makes his other assays, and no variation in moisture would influence the other and more important determinations. Some ores are sent to the smelter with from 5 to 15 per cent. of adherent water. In these cases it is best to spread out the sample, and taking equal portions fairly at regular intervals, weigh into a Berlin dish 20 grams. This should then be dried over a sand-bath, or if the ore is likely to be injured by excess of heat, over a water-bath until the weight is constant. The loss of weight multiplied by 5 gives the percentage of water present.

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