276 GENETICS 



brids of Drosera rotundifolia crossed with Drosera longi- 

 folia. The former species has 20 chromosomes, the latter 

 40. The hybrids have 30 chromosomes, 10 from rotundi- 

 folia, 20 from longifolia. In forming the germ cells of the 

 hybrid, 10 of the longifolia chromosomes conjugate with 

 the 10 from rotundifolia, leaving 10 longifolia chromosomes 

 that do not conjugate. Some of the latter die, some divide, 

 half going to each germ cell; some do not divide, but go 

 entire to one germ cell or the other. Thus the germ cells 

 vary in their chromosome numbers, from 11 to 18, in cases 

 observed. Such germ cells with irregular numbers of chro- 

 mosomes do not function properly; offspring are not pro- 

 duced from them. 



A great number of hybrids have been studied in plants 

 and animals. They show a great variety of conditions as 

 to the conjugation of the chromosomes, injury or destruc- 

 tion of chromosomes, and partial or complete sterility of 

 the hybrids, all resulting from various degrees of incom- 

 patibility of the chromosomes, combined with the occurrence 

 of different numbers of chromosomes in the two species. 



J. Incompatibility of Structures or Functions in the Two 

 Species that are Crossed. Disharmonious Combinations 



In many hybrids the chromosomes of the two species 

 are able to work together, but some of the structures or 

 functions developed in the hybrids do not operate well 

 together. The result is that the hybrids are abnormal or 

 imperfect; they may die in early stages of development. 



As an example may be taken the hybrids between two 

 species of fish, Fundulus majalis and Fundulus heteroclitus,® 

 studied by Newman (figure 56). Fundulus majalis is larger 

 than F. heteroclitus, and has larger eggs. In these large 

 eggs the circulation, after development has progressed, is 

 more rapid than in the small heteroclitus eggs, so that the 



