ASPECTS OF KINETIC EVOLUTION 2>9l 



2. Among crosses between individual clonic types, long sub- 

 jected to vegetative propagation. 



3. In a species or variety which has been placed in new and 

 unwonted conditions (neotopic mutations). 



4. Among crosses between narrowly inbred varieties (Mende- 

 lian hybrids). 



5. Among hybrids between species not too remote to combine 

 at all, but not sufficiently related to combine in a regular and 

 uniform manner. 



THE NATURE OF STERILE HYBRIDS. 



A further distinction of fundamental significance remains to 

 be added to the preceding, before the full range of the phenom- 

 ena of interbreeding can be made apparent. The general im- 

 pression has been that the development of a new individual 

 represented the result of a combination of the two parental sex- 

 cells, but this is only partially true, especially among the higher 

 plants and animals. The fusion of the parental sex-cells is 

 carried through only two of the three stages of conjugation. 

 Fertilization unites the outer, unspecialized protoplasms (plas- 

 mapsis) and also the nuclei (karyapsis), but the chromatin, 

 the most highly specialized cell-substance, the citadel, as it 

 were, of the life of the cells, remains distinct until after the new 

 individual has developed, so that the body is not composed of 

 simple, post-conjugational cells, but of double cells in a condi- 

 tion of prolonged conjugation. 



The fusion of the chromatin granules, or ultimate sex-ele- 

 ments (mitapsis), may not take place until the new individual 

 is mature and about to form new sex-cells of its own. The 

 other cells of the body never reach mitapsis. The sterility 

 of hybrids arises, it is now believed, from the inability of the 

 sex-elements to pass this third and final stage of conjugation. 

 It was always mysterious that hybrid combinations which could be 

 made for one generation could not continue for a second or a third 

 generation. This new appreciation of the nature of the process 

 of conjugation makes it apparent, however, that hybrids are 

 sterile because the parental elements do not make even one 

 complete conjugation. There is thus a definite difference 



