144 DISTRIBUTION OF THE CHROMOSOMES 



words there is expected no return to either parent 

 type, but the hybrid when backcrossed always con- 

 tinues to produce hybrids. Moreover, there is no 

 apparent weakening or other influence exerted by the 

 egg on the foreign chromosomes even in successive 

 generations. The breeding results of Standfuss, 

 who backcrossed other moths for several generations, 

 show exactly this phenomenon the same type of 

 hybrid constantly produced in every generation. 



A similar behavior of the chromosomes has been 

 recently described by Doncaster in a cross between 

 other species of moths, and is illustrated in the 

 following figures. The full number of chromosomes 

 in the moth Biston hirtaria is shown in Fig. 48, a. 

 There are 28 in all, of which four are small. Another 

 species, Biston zonaria, has something over a hundred 

 very small chromosomes (Fig. 48,6). The reduced 

 number of chromosomes of the former species is 13 

 (one large one being coupled with a small one), of 

 the latter 56. The chromosome group of the hybrid 

 (zonaria ? by hirtaria ") is shown in Fig. 48, c. The 

 exact number of chromosomes is difficult to count, but 

 there are 14 large ones and about 56 small ones. In 

 this hybrid a stage is passed through that resembles 

 the synapsis stage. When the chromosomes emerge 

 from this stage (Fig. 48, c') , almost the full number are 

 found present, although Doncaster thinks that a few 

 of them have united in pairs; for as shown in the 

 figure there are now 12 or 13 large and 50 or 51 small 

 chromosomes. These are a few less than the full 

 number present before synapsis. In this case, how- 



