484 



NA TURE 



[September 12, 1901 



offspring resulting from the union of equally ripe germ-cells dififer- 

 ing from the offspring developed from the conjugation of ripe 

 and unripe germ-cells, and still more from the union of fresh 

 and over-ripe germ-cells. This conclusion may be said to be in 

 harmony with the view expressed by Darwin, that the causes 

 which induce variability probably act "on the sex elements 

 before impregnation has been eft'ected " (" Animals and Plants," 

 vol. ii. p. 259). The results already obtained, though far from 

 answering the question why there is often great dissimilarity 

 between members of the same family, may lead to further 

 experiment, and especially to more complete records being 

 kept by breeders. It is unnecessary to point out what a gain 

 it would be were breeders able to regulate, even to a small 

 extent, the characters of the offspring. 



Is the Condition of the Soma a Cause of Variation .^ 



There is a considerable amount of evidence in support of the 

 view that changes in any part of the body or soma which affect 

 the general welfare influence the germ-cells. This is but what 

 might be expected if the soma in the metazoa is to the germ- 

 cells what the immediate surroundings are to the protozoa. The 

 soma from the first forms a convenient nidus for the germ-cells, 

 and when sufficiently old and sufficiently nourished it provides 

 the stimuli by which the ripening (maturing) of the germ-cells 

 is effected. If in the case of the protozoa variation is due to 

 the direct action of the environment, it may be inferred that in 

 the metazoa variations of the germ-cells result from the direct 

 action of the soma, i.e., from the direct action on the germ-cells 

 of their immediate environn-ent. This, however, is quite a 

 different thing from saying that definite somatic variations are 

 incorporated in the germ-cells (converted into germinal variations) 

 and transmitted to the offspring. 



It may first be asked. Does disease in as far as it reduces the 

 general vigour or interferes with the nutrition of the germ-cells 

 act as a cause of variation ? I recently received a number of 

 blue-rock pigeons from India infected with a blood parasite 

 (Halteridium) not unlike the organism now so generally 

 associated with malaria. In some pigeons the parasites were 

 very few in number, in others they were extremely numerous. 

 The eggs of a pair of these Indian birds with numerous parasites 

 in the blood proved infertile. Eggs from a hen with numerous 

 parasites fertilised by a cock with a few parasites proved fertile, 

 but the young died before ready to leave the nest. An old male 

 Indian bird, however, with comparatively few parasites mated 

 with a mature half-bred English turbit produced a single bird. 

 The half-bred turbit has reddish wings and shoulders, but is 

 otherwise white. The young bird by the Indian blue-rock is of a 

 reddish colour nearly all over, but in make not unlike the cross- 

 bred turbit hen. 



Some time before the second pair of eggs were laid the 

 parasites had completely disappeared from the Indian bird, and 

 he looked as if he had quite recovered from his long journey, 

 as well as from the fever. In due time a pair of young were 

 hatched from the second eggs, and as they approached maturity 

 it became more and more evident that they would eventually 

 present all the distinctive points of the wild-rock pigeon.' The 

 striking difference between the first bird reared and the birds of 

 the second nest might, however, be due not to the malaria 

 parasites but to the change of habitat. 



Against this view, however, is the fact that another Indian 

 bird infected to about the same extent as the mate of the half- 

 bred red turbit counted for little when mated with a second 

 half-bred turbit : while two Indian birds in which extremely few 

 parasites were found at once produced blue-rock-like birds when 

 bred — one with a fantail, the other with a tumbler. 



Another possible explanation of the dift'erence between the 

 bird of the first and the birds of the second nest is that the 

 germ-cells were for a time infected by the minute protozoan 

 Halteridium in very much the same way as the germ-cells of 

 ticks are infected by the parasite of Texas fever. But of this 

 there is no evidence, for even in the half-grown birds hatched 

 by the pure-bred malarious Indian rocks the most careful exam- 

 ination failed to detect any parasites in the blood. In all prob- 

 ability Halteridium can only be conveyed from one pigeon to 

 another by Culex or some other gnat. 



These results with pigeons suffering from malaria seem to 

 ndicate that the germ-cells are liable to be influenced by fevers 



' In these young birds the breast and some of the wing feathers are im- 

 erfect. Fanciers regard this condition of the feathers as evidence of con- 

 titutional weakness. 



NO. 1663, VOL. 64] 



and other forms of disease that for the time being diminish the 

 vitality of the parents. Further experiments may show that the 

 germ-cells are influenced in different ways by dififerent diseases. 



Sometimes the germ-cells suffer from the direct action of their 

 immediate environment, from disturbance in or around the 

 germ-glands. If, for example, inflammation by the ducts or 

 other channels reaches the germ-glands, the vitality of the germ- 

 cells may be considerably diminished ; if serious or prolonged, 

 the germ-cells may be as effectively sterilised as are the bacteria 

 of milk by boiling. 



In 1900 two mares produced foals to a bay Arab which had 

 previously suffered from a somewhat serious illness involving 

 the germ-glands. These foals in no way suggest their sire. This 

 year I have three foals by the same .\rab after he had quite 

 recovered : one promises to be the image of his sire, and the 

 other two are decidedly Arab-like both in make and action. 



While the germ-cells are liable to suffer when the soma is the 

 subject of disease, there is no evidence that they are capable of 

 being so influenced that they transmit definite or particular 

 modifications (unless directly infected with bacteria or other 

 minute organisms) ; that, e.g., the germ-cells of gouty subjects 

 necessarily give rise to gouty offspring. Doubtless if the germ- 

 cells, because of their unfavourable immediate surroundings, 

 suffer in vigour or vitality, the offspring derived from them are 

 likely to be less vigorous, and hence more likely than their 

 immediate ancestors to suffer from gout and other diseases. 



It would be an easy matter to give instances of the offspring 

 varying with the condition or fitness of the parents ; but it will 

 suffice if, before discussing intercrossing, I refer to the influence 

 of a change of habitat. 



Is Change of Habitat a Cause of Variation ? 



It has long been recognised that a change of surroundings 

 may profoundly influence the reproductive system, in some cases 

 increasing the fertility, in others leading to complete sterility. 

 Exotic plants, sterile it may be at first, often become extremely 

 fertile, and when thoroughly established give rise to new varieties. 

 In the case of mares obtained from Iceland and the south of 

 England sometimes a year elapses before they breed. An Arab- 

 Kattiawar pony which arrived during April from India proved 

 during the first three months quite sterile, owing, I believe, to 

 loss of vigour on the part of the germ-cells, their vitality being 

 only about one-tenth that of a home-bred hackney pony. But 

 the fertility is apparently greatly impaired by even comparatively 

 slight changes of environment. Lions which breed freely in 

 Dublin seem to be sterile in London, and I heard recently that 

 when bulls are changed from one district to another in the north of 

 Ireland complete sterility is sometimes the result. The tendency 

 of some exotic plants to " sport " after they become acclimatised 

 is doubtless due to the fact that their new habitat is unusually 

 favourable, their general vigour — so essential for new develop- 

 ments — is increased, and, probably because certain groups of 

 germ units are constantly stimulated by the new food available, 

 they give rise abruptly or gradually to new and it may be un- 

 expected characters. No one doubts that the bodily vigour is 

 liable to be impaired by fevers and other diseases, by changes 

 in the habitat, unsuitable food, rapid and unseasonable changes 

 of temperature, and the like ; hence it will not be surprising if 

 further investigations prove that changes in the soma, beneficial 

 as well as injurious, are reflected in the germ-cells, and thus in- 

 directly induce variation. Moreover there are excellent reasons 

 for believing that the germ-cells are influenced by seasonable 

 changes, such as moulting in birds and changing the coat in mam- 

 mals. In the case of pigeons, e.g., the young bred in early sum- 

 mer are, other things being equal, larger and more vigorous, and 

 mature more rapidly, than birds hatched in the late summer or 

 autumn. But however sensitive the germ-cells may be to the 

 changes of their immediate environment, i.e., the soma or body 

 in which they are lodged, there is no evidence whatever that 

 (as Buffon asseited and Darwin thought possible) definite changes 

 of the soma, due to the direct action of the environment, can be 

 imprinted on the germ-cells. By the direct action of the en- 

 vironment — food, temperature, moisture, &c. — the body in 

 whole or in part may be dwarfed, increased, or otherwise modi- 

 fied : but such changes only influence the germ-cells in as far 

 as they lead to modifications of the body as a whole. They 

 may expedite or delay maturity, after the length of the repro- 

 ductive period, interfere with the nutrition of the germ-cells, or 

 retard the development of the embryo, but they seem incapable 

 of giving rise to definite structural or functional variations in the 

 offspring. 



