TRANSACTIONS OF SECTION D, 671 



recovered from bis loncj 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 litth> 

 when mated with a second half-bred turbit; while two Indian birds in which 

 extremely few parasites were found at once produced blue-rook-like birds when 

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



Another possible explanation of the difference between the bird of the first 

 and the birds of the second nest, is that the germ-cells Avere for a time in- 

 fected by the minute protozoan Ilalteridium 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 halt-grown birds hatched by the pure- 

 bred malarious Indian rocks the most careful examination failed to detect any 

 parasites iu the blood. lu all probability Ilalteridium can only be conveyed from 

 one pigeon to another by Culex or some other gnat. 



These results with pigeons suffering from malaria seem to indicate that the 

 germ-cells are liable to be influenced by fevers 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 different 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 gei-m-glands. These foals in no 

 way suggest their sire. This year I have three foals by the same Arab 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 tiie 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, c.^., the germ-cells of gouty subjects necessarily' 

 give rise to gouty offspring. Doubtless if the germ-cells, because of their 

 unfavourable immediate snrroimdings, suffer in vigour or vitality, the oflTspring 

 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 inter- 

 cros.sing, I refer to the influence of change of habitat. 



7.S Chanye 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 trapses before they breed. An Arab-Kathiawar 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 



' In these young birds the breast and some of the wing feathers are imperfect. 

 Fanciers regard this condition of the feathers as evidence of constitutional weakness. 



