But Weismann puts forward his views in most sober earnest, elaborates the details of his theory, and remodifies his conceptions so that his story may fit in with the fresh discoveries in embryology and comparative anatomy as these are made. Our criticism is simply this, that the hypothesis, being nothing more than a personal conception of the author’s, is not to be tested by experiment, and that the author could always escape from the clutch of refutation. We are left, moreover, in the same difficulty with which we started, for by giving his living units the functions of growth and reproduction, which must involve heredity itself, Weismann shunts back the question from animal and plant, which we can see and handle, to particles quite beyond our ken. We may also point out that there is no reason to assume that living matter is made up of little parts, or persons, molecules or groups of molecules; experiment, in fact, strongly contradicts this assumption. Let us see how humanity first obtained the idea that matter is made up of little bits, how we obtained the idea of molecule or atom. The idea is an ancient one, and it is easy to see how it occurred to thinkers even in primitive times. Most solids, such as chalk, sand, and rock, and fluids too and gases, are, when broken or divided, of obviously similar parts. If we break a piece of writing chalk across, each fragment of it is chalk--that is, it appears to us to be white and hard, it will write upon the board, etc. If we continue to break it we shall always have smaller particles of chalk, never anything else. Thus men generalised from their commoner experiences, and got the idea that all things are built up of small parts or molecules. Anaxagoras (70th Olympiad), from experiences such as those just described, in all probability arrived at his theory of elements (homœomeriæ), viewing all things as built up of elementary things of the same nature, flesh and blood of elements of flesh and blood, etc. Similar views are held to the present day, and are an expression of the fact that you can break most things into similar but smaller parts. Quoting from Tait’s “Properties of Matter,” 1885, p. 21, we find: “But the really extraordinary fact, already known in this part of our subject, is the apparently perfect similarity and equality of any two particles of the same kind of gas, probably of each individual species of matter, when it is reduced to a state of vapour. Of such parts, therefore, whether they be further divisible or not, each species of solid or liquid must be looked upon as built up.” It will be noted that the moderns would make a distinction which the ancients did not; a modern will speak of a molecule of a gas or of incandescent iron, but he hesitates before speaking of a molecule of an ordinary solid at ordinary temperature, and would certainly, if an exact thinker, never dream of speaking of a molecule of wood or flesh or bone. If we carefully prepare as perfect a cylinder