THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 275 



major gene-differences, the whole genotypic backgrounds of the species 

 are not the same, so that the dominance relations of the same gene. 

 Crinkled Dwarf, are different when it is introduced into the different 

 species (p. 298). 



B. CYTOPLASMIC DIFFERENCES 



I. Inheritance through the Cytoplasm} 



It has often been suggested that organisms which are far removed 

 from one another taxonomically may differ not only in the genetic 

 characters borne in their chromosomes but also in the nature of their 

 cytoplasm. In the nature of the case it is difficult to test this hypothesis 

 directly, since widely different organisms cannot be crossed. It is, 

 however, certain, if only on embryological grounds, that in many cases 

 the divergence of species during evolution carries with it a divergence 

 in the nature of the egg-cytoplasm. This does not justify, however, the 

 common deduction that evolution is carried out by means of alterations 

 of the cytoplasm and that the differences studied by geneticists are 

 therefore of little importance for it. The evidence, as far as we know it, 

 is that the differences between nearly related species are usually entirely 

 chromosomal, so that one must suppose that the first steps in species- 

 divergence are caused by alterations in the genes, and that the cyto- 

 plasmic differences only become developed at later stages in the 

 process. 



It is probable, in fact, that in nearly all species the characteristics of 

 the egg-cytoplasm are entirely dependent on the activities of the chromo- 

 somes. It is only in comparatively few cases, which will be reviewed 

 below, that we can discover cytoplasmic factors which can be per- 

 petuated in the absence of the appropriate chromosomes. 



Cytoplasmic differences are revealed in the comparatively rare cases 

 in which the result of a cross depends on the way it is made; that is, 

 when the offspring resemble the mother, whichever species the mother 

 may be. One of the best examples in higher organisms is in the willow 

 herb Epilobium,^ in which, even after fourteen generations of crossing 

 E. luteum ? x £". hirsutum (^ the influence of the luteum plasma 

 could be clearly seen. Another example is the inheritance of male 

 sterility in fiax^ and maize,* and the rather similar inheritance of the 

 female factor through the c5^oplasm in Lymantria (p. 216). 



^ Rev. East 1934, Goldschmidt 19346, Pellew 1929, Wettstein 1937a. 



2 Cf. Michaelis 1937. ^ Gairdner 1929. * Rhoades 1935. 



