I should prefer not to make any definite statement about our problem at present. Our actual knowledge of the organisation and metabolism of both nucleus and protoplasm is so extremely small and may relate to such very insignificant topics, that any definite decision is impossible. I myself believe that the nucleus plays an important part in heredity, perhaps even a greater one than protoplasm, but this is only my belief.[136]
The discovery of Gruber and others, that Protozoa are only capable of restitution if they contain at least a fragment of the nucleus, has also been used occasionally as a proof of the morphogenetic importance of the nucleus. But might not this absence of restitution where nuclear material is lacking be understood equally well on the hypothesis of Loeb and R. S. Lillie that the nucleus is a centre of oxidation in the cell? Remove the heart from a vertebrate and the animal will not digest any more; but in spite of that the heart is not the organ of digestion.
And so we lay stress once more upon this point: that the experimental results of hybridisation and the analytical results obtained by the discussion of the complex-equipotential systems are of greater value to the theory of heredity than all speculation about the importance or unimportance of special constituents of the cell, of whose organisation, chemistry, and physics, scarcely anything is known at present.[137]
Variation and Mutation
Heredity, it has been said, may be understood as resting upon the fact that each organism forms its own initial stage again, and that this initial stage always encounters conditions of the same kind.
If this statement were quite correct, all the individuals of a given species would be absolutely alike everywhere and for ever. But they are not alike; and that they are not alike everywhere and for ever is not merely the only real foundation of the so-called theory of descent we possess, but also forces us to change a little our definition of heredity, which now proves to have been only a sort of approximation to the truth, convenient for analytical discussion.
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.
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]