208 THE LAW OF JOHANNSEN. 



look upon as causing this diversity, is limited to the proto- 

 plasm outside the nucleus, to the cytoplasm exclusively. 



All the recent observations on the effect of selection in 

 clones of uni-cellulars point to this same conclusion, namely, 

 that there is a very great difference between an inheritance, a 

 transmission of substances, genes, through the cytoplasm on 

 one side, and through the nucleus on the other. We know that 

 there is a list of cases of more or less pronounced maternal 

 inheritance. In some of these cases the mechanism is clear, 

 such as in the cases of infectious spotting in plants, and even 

 in the cases of the transmission of self-propagated corpuscles 

 outside the nucleus. In other cases the preponderance in bulk 

 of the cytoplasm in the gamete derived from the mother, 

 makes the explanation simple. 



It is certainly significant, that we are gradually eliminating 

 the alleged instances of non-Mendelian inheritance, and that 

 these instances are rapidly being restricted to cases of a contin- 

 uation of cytoplasmic conditions through germ-cells, or 

 through whole embryos. 



A new distribution of the genes over the nuclei, occurs al- 

 most exclusively, at- cell-divisions preparing for sexual repro- 

 duction. Exceptions are found, such as instances of somatic 

 segregation in pure clones (Salaman), but they can generally 

 be recognized as such. 



So long as there is no such process in a given material, 

 we can say that there is no genetic variability in it. Ordinarily 

 there is no genetic variability in pure clones, in groups which 

 have been derived by exclusive vegetative reproduction from 

 one zygote. And there is no genetic variability in what Jo- 

 hannsen has called a pure line, in material which is derived by 

 exclusive self-fertilization from one homozygous plant. In such 

 material by definition all gametes produced are identical, and 

 all zygotes must be, so that (with the exception of the case of 

 a spontaneous loss) all the nuclei in such a material contain 

 the same set of genes. 



Johannsen observed, that selection in material which was 



