668 REPORT— 1901. 



and instincts, and in the aptitude to learn. In the members of a human family 

 there may be great dissimilarity, and the dissimilarity may be even greater in the 

 members of a single brood or litter of domestic animals, especially if the parents 

 belong to slightly different breeds. 



Frequently some of the offspring closely resemble the immediate ancestors, 

 "while others suggest one or more of the remote ancestors, are nearly inter- 

 mediate between the parents, or present quite new characters. Similarly 

 seedlings from the same capsule often difler. Can we by way of accounting 

 for these differences only with Darwin say that variations are due to fixed and 

 immutable laws, or at the most subscribe to the assertion of Weismann, that 

 they are 'due to the constant recurrence of slight inequalities of nutrition 

 of the germ-plasm'?^ Weismann accounts for ordinary variation by saying 

 that the reduction of the germ-plasm during the maturation of the germ-cells 

 is qualitative as well as quantitative, i.e., that the germ-plasm retained in 

 the ovum to form the female pro-nucleus is different from the germ-plasm dis- 

 charged in the second polar body. He accounts for discontinuous variation and 

 ' sports ' by ' the permanent action of uniform changes in nutrition.' ^ These 

 uniform changes in nutrition, by modifying in a constant direction susceptible 

 groups of germ-units (determinants), after a time giving rise to new, it may be 

 pronounced variation. Must we rest satisfied with these assumptions, or is it 

 possible to account for some of the variability met with by, say, differences in the 

 maturity of the parents or of the germ-cells, by the germ-cells having been 

 influenced by interbreeding or intercrossing, or by the soma in which they are 

 lodged having been invigorated by a change of food, or habitat, or deteriorated by 

 unfavourable surroundings or disease ? In other Avords are there valid reasons 

 for believing that the germ-cella are extremely sensitive to changes in their 

 immediate environment, i.e., to modifications of the body, or soma containing 

 them, and that the characters of the offspring depend to a considerable extent on 

 whether the germ-cells have recently undergone rejuvenescence ? 



Obviously, if the offspring, other things being equal, vary with the age of the 

 parents, the ripeness of the germ-cells and with the bodily welfare, the qualitative 

 division of the nucleus on which Weismann so much relies as an explanation of 

 ordinary variation will prove inadequate. 



Is Age a Cause of Variation? 



During the course of my experiments on Variation I endeavoured to find an answer 

 to the question, ' la Age a Cause of Variation ? ' During development and while 

 nearly all the available nourishment is required for building up the organs and 

 tissues of the body, the germ-cells remain in a state of quiescence. Sooner or 

 later, however, they begin to mature, and eventually in most cases escape from the 

 germ-glands. I find the first germ-cells ripened often prove infertile. "When, 

 e.g., pigeons from the same nest are isolated and allowed to breed as soon as 

 mature, they seldom hatch out birds from the first pair of eggs, and though 

 quite vigorous in appearance they may only hatch a single bird from the second 

 pair of eggs. The same result generally follows mating very young but quite 

 tmrelated pigeons ; but when a young hen bird is mated with a vigorous, well- 

 matured male, or a young male is mated with a vigorous, well-matured female, 

 the eggs generally prove fertile from the first. The germ-cells are, as far as ca.n 

 be determined, structurally perfect from the outset ; and that they only fail in 

 vigour is practically proved by the fact that, though the conjugation of germ-cells 

 from two young birds leads to nothing, the conjugation of germ-cells from quite 

 young birds with germ-cells from mature birds generally at once results in 

 offspring. 



The following experiments indicate how age may prove a cause of variation. 

 Last autumn I received from Islay two young male blue-rock pigeons which, 

 though bred in captivity, were believed to be as pure as the wild birds of the 

 Islay caves. In February last one of the young blue-rocks, while still immature, 



' Germ-Plasm, . 431, 



