DISTRIBUTION OF THE CHROMOSOMES 191 
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- 
