134 W. J. MOENKHAUS 
In so far as the above determination may be taken as a measure 
of the vitality of this species we are justified in concluding that 
from six to seventeen generations of inbreeding no appreciable 
deterioration has resulted. No such exact determinations were 
made in later generations, and it is possible that eventually the 
effects of inbreeding would manifest themselves, but my observa- 
tions during seventy-five or more generations does not lead me 
to believe this. 
4. Sterility and selection 
Along at the thirteenth and fourteenth generations the sterility 
had become very pronounced. Of the offspring of some of the 
pairs, more than 50 per cent of the males were sterile. On the 
other hand, while practically all pairs showed at least some degree 
of sterility this varied very much in the different brothers and sis- 
ters of the same brood. That this sterility was a direct physiolog- 
ical result of the inbreeding seemed to me very doubtful. To 
find the effects of inbreeding showing itself in such a specific way 
upon the males only, did not, to say the least, meet expectations. 
Furthermore, sterility was not wholly wanting in forms that had 
not been inbred. 
It was highly desirable to continue the experiments on inbreed- 
ing, and yet to keep the strain alive, it was necessary to find some 
way to eliminate this high degree of sterility. The process that 
was most effective was selection. By continuing the strain of 
those pairs whose offspring showed the highest degree of fertility 
but at the same time continuing the rigorous inbreeding, it was 
possible almost completely to eliminate the sterility. This at the 
same time gave one of the severest tests as to whether inbreeding 
was the responsible factor, for if the sterility could be eliminated 
by continuing the very process of inbreeding the latter could not 
well be held to be the cause of it. 
This was done as follows: In the fourteenth generation three 
fertile pairs of brothers and sisters from the same brood were iso- 
lated and mated. The offspring of each of these were mated in 
pairs to determine the degree of sterility. By reference to table 
