GENERAL RELATIONS IN INHERITANCE 309 



stock or race, all members of which have the same genetic 

 constitution. Such a race may continue to multiply un- 

 changed for many generations or indefinitely. 



This occurs in Protozoa, in which conjugation gives the 

 new combinations, while fission multiplies them. It occurs 

 in Hydra, through budding after biparental reproduction, 

 and in many other lower animals: the Coelerrterates, worms, 

 ascidians. In certain other groups, as the mollusks, arthro- 

 pods and vertebrates it is less common; in the higher verte- 

 brates it hardly occurs save in the rare production of iden- 

 tical twins. In these groups reproduction occurs mainly by 

 the union of germ cells, giving in each generation new com- 

 binations of genes. In plants vegetative reproduction occurs 

 on a grand scale, multiplying the combinations produced 

 in biparental reproduction. 



In many organisms there is another type of uniparental 

 reproduction, known as parthenogenesis. The female pro- 

 duces ova which develop without fertilization, so that there 

 is no combination of genes from two parents. In some cases 

 of parthenogenesis, a reduction in the number of chromo- 

 somes (and genes) occurs in forming the ova, so that the 

 offspring produced are haplold. Haploid individuals so 

 produced may differ in their genetic constitution from the 

 diploid parent. Thus, a diploid parent with genes AaBb, 

 etc., could by parthenogenesis produce haplold offspring of 

 the different types AB, Ab, aB, ab, and so on. But when 

 parthenogenesis continues after the haploid condition is 

 reached, there is no further change in the gene combina- 

 tions. In some other cases of parthenogenesis, the diploid 

 condition is restored by reunion of one of the polar bodies 

 with the ovum. This may give rise to gene recombinations 

 for a generation or two. But as parthenogenesis continues 

 for generation after generation, changes In the gene com- 

 binations cease. In such organisms as Rotifera and the lower 

 Crustacea, there Is usually a short period of sexual or bi- 



