Probably the most interesting of all his duties to the analyst is that of judging from what animal the blood stains came. This can be done only in some cases; that is, when the blood is not quite so old that the red corpuscles have entirely lost their shape. Of course this is a matter of the greatest importance when a man is on his trial; for, in the first place, every spot of blood found on his belongings is supposed to have come from his victim, although it may be nothing more than the blood of a fish; and, in the second place, the stock explanation of blood stains on his clothing offered by a prisoner is that they came from some animal he killed. The plan is to ask him what animal. Five times out of six he will say a domestic fowl or some kind of bird especially if he is a poacher who has killed a gamekeeper--and then he is done for. Look at the pictures on page 149 and you have the whole thing in a nutshell. It will be seen that the red corpuscles of the blood of birds, reptiles, and fishes (with the exception of the cyclostomata) are oval, while those of mammalian blood are round. Here is, at once, a sure way of differentiating mammalian blood from that of the other three great classes of animals. The only difficulty is that blood corpuscles get out of shape, under certain circumstances, and are no longer either oval or round. But there is another difference. A mammalian corpuscle is of uniform substance throughout: that of a fish, bird, or reptile has a small, dense spot near the centre, called a nucleus. Snails, slugs, worms, and other low forms of animal life do not come into the question at all, for their blood is generally colourless, and, if not, it is blue-green, violet, brown, being scarcely ever red, and then not from the presence of corpuscles. All that remains for the analyst, therefore, supposing he finds a round corpuscle, is to say to what mammalian animal it belongs. (The llama, alpaca, camel, and their kin, by the way, have oval corpuscles.) How are the corpuscles of different mammalia to be distinguished under the microscope? Merely by their size. They have all been measured with the greatest care, a specially small unit of length, called a micron, having been invented for the purpose. It is only 1/25000 of an inch long, and, expressed in tenths of a micron, the average diameter of a human blood corpuscle is 77; of a dog, 73; of a rabbit, 69; of a cat, 65; of a sheep, 50; of a goat, 41; and of an elephant, 94. But these are average measurements, and some corpuscles are smaller, some larger. [Illustration: MORE TINY CLUES.--HUMAN HAIR CONTRASTED WITH ANIMALS' HAIR, WOOL, AND FIBRE. Cat's Hair. Bat's Hair. Berlin Wool. Reindeer's Hair. Woody Fibre. Human Hair. Fox Hair. Hare's Hair. Squirrel's Hair. Human Hair Bulb.]