September 12, 1901] 



NA TURE 



48; 



Frequently some of the offspring closely resemble the imme- 

 diate ancestors, while others suggest one or more of the remote 

 ancestors, are nearly intermediate between the parents, or pre- 

 sent quite new characters. Similarly seedlings from the same 

 capsule often differ. Can we by way of accounting for these 

 differences only with Darwin say 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"? (" Germ- 

 Plasm," p. 431). Weismann accounts for ordinary variation by 

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

 of the germ-cell is qualitative as well as quantitative, i.e. 

 that the germ-plasm retained in the ovum to form the female 

 pro-nvicleus is different from the germ-plasm discharged in the 

 second polar body. He accounts for discontinuous variation and 

 "sports" by "the permanent action of uniform changes in 

 nutrition " (" Germ-Plasm," p. 431). 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 in- 

 fluenced 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 words, are there valid reasons for believing 

 that the germ-cells 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 en- 

 deavoured to find an answer to the question, ** Is 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 quies- 

 cence. 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 unrelated 

 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 can be determined, 

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

 vigour is practically proved by the fact that though the con- 

 jugation 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 be- 

 lieved to be as pure as the wild birds of the Islay caves. In 

 February last one of the young blue-rocks, while still immature, 

 was placed with an inbred white fantail, the other with an ex- 

 tremely vigorous well-matured black barb. In course of time a 

 pure-white bird was reared by the white fantail, and two dark 

 birds by the black barb. Owing probably to the fantail being 

 inbred and the blue-rock being still barely mature, the young 

 white bird died soon after leaving the nest. No birds were 

 hatched from the second and third pairs of eggs laid by the fan- 

 tail, but from the fourth pair two birds were hatched which are 

 now nearly full-grown. These young birds are of a darker shade 

 of blue, and look larger and more vigorous than their blue-rock 

 sire. As in the Indian variety of the blue-rock pigeon the croup 

 is blue, and, as in some of the Eastern blue-rocks, the wings are 

 slightly chequered. They, however, only essentially differ from 

 their sire in having four extra feathers in the tail. The first pair 



NO. 1663, VOL. 64] 



of birds hatched by the black barb when they reached maturity 

 early in August might have passed for young barbs with some- 

 what long beaks. Since the first pair were hatched in March 

 the blue-rock and black barb have reared six other birds. One 

 of the second brood closely resembles the first birds hatched ; 

 the other is of a greyish colour, with slightly mottled wings, a 

 long beak, and a tail bar. The birds of the third nest are both of 

 a greyish colour, but have indistinct wing bars as well as a tail 

 bar. Of the fourth pair of young, one is greyish like the birds of 

 the third nest, the other is of a dark blue colour with slightly 

 chequered wings, and a head, beak and bars as in its blue-rock sire. 

 The gradual change from black to dark blue in the blue-rock barb 

 crosses is very remarkable. I can only account for the almost 

 mathematical regularity of the change by supposing it has kept 

 pace with a gradual increase in the vigour or prepotency in the 

 young blue-rock. Eventually the offspring of the blue-rock 

 mated to the black barb, like the offspring of its brother with 

 the white fantail, may be of a slaty blue colour, and otherwise 

 resemble a wild blue-rock pigeon. Many breeders would 

 explain the offspring taking more and more after the sire by the 

 doctrine of Saturation — a doctrine that finds much favour 

 amongst breeders — but as identical results were obtained when 

 young females were mated with well-matured males the 

 saturation explanation falls to the ground. 



Like results were obtained by breeding young grey quarter- 

 wild rabbits with an old white Angora buck : the first young 

 were white, the subsequent young were white, grey and bluish 

 grey. From these results it follows that when old and young 

 but slightly different members of a variety or species are marked 

 a wonderfully perfect series of intermediate forms is Ukely to be 

 produced. Amongst wild animals the young males rarely have 

 a chance of breeding with the young females ; hence amongst 

 wild animals, owing to age being a cause of variation, a con- 

 siderable amount of material is doubtless constantly provided 

 for selection, thus affording a variety an additional chance of 

 adapting itself to slight fluctuations in the environment. 



In the results obtained by crossing mature, vigorous, and, in 

 some cases, inbred males with barely mature females, an explana- 

 tion may be found why in some families the same features have 

 persisted almost unaltered for many generations ; why in his 

 features the squire of to-day sometimes exactly reproduces the 

 lines of his ancestors, as seen in portraits and monumental 

 brasses. It should, however, be borne in mind that highly pre- 

 potent forms are capable from the first of so completely con- 

 trolling the development that they transmit their peculiar traits 

 to all their offspring. 



Is Ripeness of the Germ- Cells a Cause of Variation ? 



While difference in age may sometimes account for the earlier 

 broods and litters resembling [one of the parents, it fails to 

 account for the very pronounced variation often found in a single 

 brood or Utter, and for much of the dissimilarity between 

 members of the same human family. When a single fertilised 

 germ-cell, as occasionally happens, gives rise to twins, they are 

 always identical ; hence it may be assumed the differences in 

 members of the same family have their source in differences in 

 the germ-cells from which they spring. If the offspring vary 

 with the maturity of the soma, it may also vary with the maturity 

 of the germ-cells, or at least with their condition at the moment 

 of conjugation. 



Some years ago Mr. H. M. Vernon, when hybridising echino- 

 derms, discovered that " the characteristics of the hybrid off- 

 spring depend directly on the relative degrees of maturity of 

 the sexual products " (Proceedings Roval Society, vol. Ixiii. May 

 1S98). Mr. Vernon found subsequently that over-ripe (stale) 

 ova fertilised with fresh sperms gave very different results from 

 fresh ova fertilised with over-ripe (stale) sperms, from which he 

 inferred that over-ripeness (staleness) is a very potent cause of 

 variation {ibid., vol. Ixv. November 1S99). 



I find that if a well-matured rabbit doe is prematurely (i.e., 

 some time before ovulation is due) fertilised by a buck of a 

 different strain, the young take after the sire ; when the fertilisa- 

 tion takes place at the usual time, some of the young resemble 

 the buck, some the doe, while some present new characters or 

 reproduce more or less accurately one or more of the ancestors. 

 When, however, the mating is delayed for about thirty hours 

 beyond the normal time, all the young, as a rule, resemble the 

 doe. It may hence be inferred that in mammals, as in echino- 

 derms, the characters of the offspring are related to the con- 

 dition of the germ-cells at the moment of conjugation, the 



