CHAPTER XXIV 



GENETIC CHANGES IN ASEXUAL REPRODUCTION IN 

 PARTHENOGENESIS, AND IN SELF-FERTH^IZATION 



The frequency of occurrence of variation in single genes ap- 

 parently is very different in different species of animals and 

 plants, and in different modes of reproduction. It is sup- 

 posed to be commonest in organisms which reproduce only 

 sexually but it must be remembered that sexual reproduction 

 favors the spread of any genetic change which happens to 

 occur, whereas under asexual reproduction a mutation, how- 

 ever favorable, has no chance to spread from the family in 

 which it originated to others of the same species. Accord- 

 ingly mutation (in single genes) may seem to be less common 

 than it really is, in organisms which are propagated asexu- 

 ally. It is only w^hen systematic search is made for genetic 

 variations that we gain any adequate idea of how com- 

 monly they occur. Jennings was the first to take this matter 

 up in connection with the asexual reproduction of the pro- 

 tozoan, Paramecium. He was unable to detect any genetic 

 changes in size in races of Paramecium reproducing by fission, 

 but in a soft-bodied animal like Paramecium in which body 

 size is constantly changing, measurement of size is not an 

 easy matter. Later Jennings sought more favorable ma- 

 terial for study and apparently found it in a shelled proto- 

 zoan, Difflugia. This has a definiteness and rigidity of form 

 which is wanting in Paramecium. Its shell can be measured 

 with great exactness and the number of spines which it bears 

 can be counted, and their length measured. In the case of 

 Difflugia Jennings found that differences in size, number of 

 spines, and length of spines may be observed among the 

 asexually produced descendants of a single individual tliat 

 in consequence of selection these differences become strength- 

 ened and divergent races are thus created. Hegner has ob- 



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