FERTILISATION 219 



mately equal quantities of each sample of semen were then mixed 

 together in a glass tube. After a further examination of the mixture, 

 when it was observed that all the sperms were still active, the fluid 

 was injected into a pure-bred Dandie Dinmont bitch, which was 

 distantly related to the Dandie Dinmont dog. Previously to the 

 experiment the bitch had been kept apart from other dogs, and this 

 restriction was continued so long as she showed signs of oestrus. 

 Fifty-nine days after the injection the bitch littered four pups, 

 which closely resembled one another. Of these one died early, but 

 the other three grew into mongrels which somewhat resembled the 

 terrier sire, so that there can be little doubt that all four pups 

 were mongrels. No stress should be laid upon the result of a single 

 experiment; but the evidence, such as it was, was indicative of a 

 selective tendency, consequent upon a reduced vitality, on the part 

 of the ova of the in-bred animal to conjugate with dissimilar rather 

 than with related spermatozoa. 



Professor Ewart has informed the writer of a case in which a 

 Dandie Dinmont bitch in his possession copulated with a dog 

 belonging to the same breed, and two days subsequently had inter- 

 course with a Scotch terrier. In due time the bitch littered three 

 pups, and of these only one was a pure-bred Dandie Dinmont, while 

 the other two were half-bred Scotch terriers, in spite of the fact 

 that the Dandie Dinmont dog copulated two days earlier than the 

 Scotch terrier. This case may be regarded as to some extent 

 confirmatory of the experiment described above. 1 



Doncaster, 2 in describing his experiments on Echinoid hybridisa- 

 tion, states " that cross-fertilisation is assisted by conditions which 

 tend to reduce the vitality of the eggs." This artificial reduction 

 of vitality could be accomplished either by warming the eggs, or by 

 shaking them, or by keeping them for several hours, or by placing 

 them for from one to two hours in diluted sea-water, the last method 

 being the most uniformly conducive to the occurrence of cross- 



1 Seeing that an assortative mating of gametes can probably occur between 

 the ova of one individual and the spermatozoa derived from different individuals, 

 whether as a result of gametic similarity or reduction of vitality, it is not 

 improbable that gametic selection also sometimes takes place when various 

 gametes of a single individual are bearers of different characters, in the manner 

 postulated by the Mendelian theory. Such a preferential mating, if it exists, 

 would of course obscure the evidence of that very gametic segregation, the 

 probable existence of which, in other cases, is inferred from the numerical 

 proportions in which the different sorts of zygotes or offspring are produced ; 

 for if there is an assortative mating among the gametes, it is obvious that the 

 offspring would no longer be produced in definite Mendelian proportions, since 

 these depend upon the chance unions of gametes. According to this view, 

 prepotency may perhaps be interpreted as the tendency of the gametes of an 

 individual to conjugate with other gametes bearing similar hereditary 

 characters. 



2 Doncaster, loc. cit. 



