NATURE AND CAUSES OF VARIATION 31 



our ignorance of the structure and nature of the germ 

 plasm. Chemists are still far from being able to tell 

 us what Hving substance is, and until we know tliis, 

 we shall probably never find out how changes in the 

 germ plasm are brought about. 



Quite recently certain investigators have succeeded 

 in producing mutations at will. The American experi- 

 menter Tower has produced a number of new types of 

 a certain beetle by subjecting the creatures or their 

 eggs to exceptional conditions of temperature and the 

 like. Macdougall, another American investigator, has 

 caused mutations in plants by injecting chemicals into 

 the seed-cup at the time of the formation of the seeds. 

 More experiments, however, Mali require to be carried out 

 before any general statement on tliis subject can be made. 



Before concluding this chapter we may discuss briefly 

 two of the disputed questions of heredity. 



The first of these is Tdegony, which is described as 

 the supposed influence on the progeny of a female of a 

 sire with wliich she had previously been mated. If, 

 for instance, a pure-bred bitch is accidentally mated 

 with a mongrel, it is said that she is spoiled for further 

 breeding. That is, her future oiispring by a male of 

 her own breed -^ill tend to partake of the mongrel 

 character. This behef is widespread among practical 

 breeders. For instance, many sheep-breeders' societies 

 have a rule to the effect that a ewe which has been 

 crossed with a ram of a different breed can no longer 

 be considered pure bred, and none of her subsequent 

 progeny can be registered. As regards the explanation 

 of telegony, if it be a fact, it is supposed that the young 

 developing animal exerts some subtle influence on the 

 constitution of its dam. But the mechanism of this 

 influence we cannot even imagine,. 



The classical example of telegony is that of Lord 

 Morton's mare, which was fully described by Darwin. 

 The mare was mated with a quagga and bore a colt. 

 Afterwards she was mated with a black Arab stallion 

 and bore two colts in succession showing distinct stripe 

 markings. 



