Experimental Contributions to the Theory of Heredity. 245 



rule results in the subsequent offspring " throwing back " either to the 

 ancestors of the sire or the clam, it will be extremely difficult, if not 

 in many cases impossible, to distinguish telegony from simple rever- 

 sion.* 



But though " infection," if it does take place, is likely, as a rule, to 

 lead the subsequent offspring to resemble the ancestors of the dam, it 

 may in certain cases possibly lead to their " throwing back" to a pre- 

 vious sire. This result might follow if the previous sire happened to 

 be highly prepotent. For example, Highland heifers often produce to a 

 Galloway bull hornless black offspring indistinguishable from pure 

 Galloways. If infected by the Galloway bull, these heifers might 

 afterwards produce Galloway-like calves when mated with long-hornecl 

 bright coloured bulls of their own breed. 



It is now commonly believed that if there is such a thing as telegony 

 it results from the unused germ cells of the first (or previous) sire 

 infecting — blending with — the unripe germ cells in the ovaries of the 

 dam. Were this possible, the subsequent progeny would in all proba- 

 bility in a mild way resemble the previous sire, but if this is impos- 

 sible, then infection — due perhaps to some obscure change in the con- 

 stitution or reproductive system of the dam — is more likely to lead to 

 more or less marked reversion to the ancestors of the dam. All my 

 observations point to its being impossible in the Equidse for the 

 unused male germ cells of the first sire to infect the unripe ova. The 

 spermatozoa lodged in the upper dilated part of the oviduct of the 

 mare are dead, and in process of disintegrating, eight days after insem- 

 ination ; they probably lose their fertilising power in four or five days. 

 There is no reason for supposing that in the Equidse they survive 

 longer in or around the ovary. Further, though at the time of fertili- 

 sation there may be several large Graafian follicles in each ovary con- 

 taining maturing ova, all these follicles disappear long before the 

 period of gestation is completed. The subsequent foals are developed 

 from successive new crops of ova into the composition of which it is 

 inconceivable any of the spermatozoa of the first sire could by any 

 chance enter. A study of the ovaries hence tends to confirm the view 

 that " infection " (if there is such a thing) is as likely to cause rever- 

 sion to a former ancestor of the dam as a " throwing back " to a pre- 

 vious sire. 



Having made these general observations, it will be well next to con- 

 sider critically the case of " infection " communicated in the letter to 

 the President of the Eoyal Society in 1820 by the Earl of Morton. 



* That reversion ever occurs has been questioned by Bateson (' Materials for 

 the Study of Variation') and others, but I have already (' Mature,' February 9, 

 1899) proved beyond doubt that reversion can be easily induced by intercrossing 

 distinct types, and I have recently heard of several instances of spontaneous reversion 

 — reversion not induced by intercrossing. 



