198 THE THEORY OF THE GENE 



new stable hybrid that is fertile has been produced by 

 crossing two species with widely different chromosome 

 numbers. Ljungdahl (1924) crossed Papaver nudicaule, 

 having 14 chromosomes (n=7), with P. striatocarpum, 

 with 70 chromosomes (n=35) (Fig. 111). The hybrid has 

 42 chromosomes. At maturation of the hybrid germ-cells 

 there are 21 bivalents (Fig. Ill, b, c-e). These divide, 21 

 going to each pole. No single chromosomes are present, 

 and none lag on the spindle. The result must be inter- 

 preted to mean that the 7 chromosomes of nudicaule have 

 mated with 7 chromosomes of striatocarpum, and that the 

 remaining 28 chromosomes of striatocarpum have con- 

 jugated in twos to give 14 bivalents. This gives a total of 

 21 bivalents, the number observed. It seems natural to 

 assume that the form striatocarpum, with 70 chromo- 

 somes (n=35), is probably a decaploid type, i.e., a type 

 with ten times each kind of chromosome. 



The new type (F x ) produces germ-cells with 21 chro- 

 mosomes. It is balanced and stable. It is also fertile and 

 may be expected to produce a new stable type. From it 

 still other stable types are theoretically possible. If back- 

 crossed to nudicaule it should give rise to a tetraploid 

 type (21+7=28). Back-crossed with striatocarpum it 

 should produce an octoploid type (21+35=46). Here, 

 through hybridization of a diploid and a decaploid type, 

 there may be produced in subsequent generations tetra- 

 ploids, hexaploids, and octoploid types that are stable. 



Federley's experiment (Chapter IX) with species of 

 moths of the genus Pygaera illustrate a very different 

 relation. Owing to the failure of the chromosomes to con- 

 jugate in the germ-cells of the hybrid the double number 

 is retained. By back-crossing the double number may be 

 continued, but as the hybrids are very sterile nothing 

 permanent could result from these combinations under 

 natural conditions. 



