In the first place, the conditions which surround the initial stages of morphogenesis are not quite equal in every respect: and indeed the offspring of a given pair of parents, or better, to exclude all complications resulting from sexual reproduction, or amphimixis, as Weismann called it--the offspring of one given parthenogenetic female are not all equal among themselves. The individuals of each generation are well known to vary, and it is especially in this country that the so-called individual or fluctuating variation has been most carefully studied by statistical methods, Galton and Weldon being the well-known pioneers in this field.[138] In fact, if we are allowed to assume that this sort of variation is the outcome of a variation of conditions--in the most general meaning of the word--we only follow the opinion which has almost universally been adopted by the biologists[139] that are working at this branch of the subject. Variation proper is now generally allowed to be the consequence of variations in nutrition; the contingencies of the latter result in contingencies of the former, and the law of contingencies is the same for both, being the most general law of probability. Of course under such an aspect fluctuating variation could hardly be called an exception, but rather an addition to inheritance. [138] H. M. Vernon, *Variations in Animals and Plants*, London, 1903. [139] De Vries, *Die Mutationstheorie*, i., 1901; and Klebs, *Jahrb. wiss. Bot.* 42, 1905. But there are other restrictions of our definition of heredity. The initial stage which is formed again by an organism is not always quite identical in itself with the initial stage of its own parent: Bateson and de Vries were the first to study in a systematic way these real exceptions[140] to true inheritance. As you know, de Vries has given them the name of “mutations.” What is actually known on this subject is not much at present, but nevertheless is of great theoretical value, being the only real foundation of all theories of descent, as we shall see in the next lectures. “Mutations” are known to exist at present only among some domesticated animals and plants. Nothing of a more general character can be said about their law or meaning.[141] [140] They would not be “real exceptions” if Klebs (*Arch. Entw. Mech.* 24, 1907) were right in saying that both variations and mutations owe their existence to external agents. What is really *proved* by Klebs is the possibility of changing the *type* of a curve of variation and of provoking certain discontinuous varieties by external means. See also Blaringhem (*Comptes rend.* 1905-6, and *Soc. de Biol.* 59, 1905), and MacDougal (*Rep. Depart. Bot. Res., 5th Year-book Carnegie Inst.*, Washington, 129).