THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 233 



sink to one kind of id, that is, that the germ-plasm may then consist 

 entirely of identical ids. If chance variations of certain determinants 

 in unfavourable directions occur in some of the ids composing the 

 germ-plasm, these are brought together in the offspring from both 

 the maternal and the paternal side, and will occur in an increasing- 

 number of ids the longer the inbreeding has gone on, that is, the 

 smaller the number of different ids has become. The unfavourable 

 variation- tendency is therefore persisted in, and its influence upon 

 the development of a new descendant will be the greater the larger 

 the number of identical ids with these unfavourable variations. It is 

 obvious that the crossing of an animal, which is thus, so to speak, 

 degenerating slightly, with a member of an unrelated family must 

 immediately have a good effect upon the descendants, for in this 

 way quite different ids with other variations of their determinants 

 are introduced into the inbred germ-plasm which had become too 

 monotonous. 



From this theoretical interpretation of the injurious consequences 

 of inbreeding we may at once infer that not every inbreeding 

 necessarily implies degeneration, for the occurrence of unfavourable 

 variational tendencies in the germ-plasm is presupposed as the 

 starting-point of degeneration, and if these do not exist there can be 

 no deo'eneration. This harmonizes with the fact that the evil effects 

 of inbreeding are observed to vary greatly in amount, and may not 

 occur at all. But they are greatest in breeds artificially selected 

 by man, which have long been under unnatural, directly influential 

 conditions, and are also removed from the purifying influence of 

 natural selection. In such cases, therefore, there is every probability 

 that diverse unfavourable variational tendencies in the determinants 

 will occur. 



But how are we to understand the fact that pure parthenogenesis 

 may last through innumerable generations, and yet no degeneration 

 sets in "? I believe very simply. In this case too, the same ids which 

 were peculiar to the mother of the race are contained in the descend- 

 ants, but they do not diminish in number, for in pure and normal 

 parthenogenesis, such as that of Cypris rei^tans, the second maturation- 

 division of the ovum does not take place, and this is precisely the 

 nuclear division which effects reduction. In addition, the introdiiction 

 of identical ids, which must take place in the case of inbreeding 

 at every amphimixis, does not occur, and, what is certainly of great 

 importance, all these cases are old species, living under natural 

 conditions — the same conditions under which they lived as amphi- 

 gonous .species, and not newly formed breeds under artificial 



