Let us briefly exaxtne the main caAegort8s of the currently available

 
evidencs. 
 
     (I)            -1--,~ CF        OuZ ~L~~S ~r Iowa data on nuabers of
young 
 
muasrats conceived or born in nearly 1200 litters, 1935-4T, show a decrease

 
from an avermwe sie of 6.6 for 1935 to an average of 6,3 for    , followed

 
by a generally gradual Increase in average sise to a.2 for 141 and 8.5 for

 
1942, then by a dareao e to 6.k for 1,4 andi an as yet iwperfectly determined

 
increae for 1947. The years of tVe mller and the larger average sies 
 
of litters crrespond reapectivly to what may be regarded as the loa and 
 
high phases of the &muse and hare cyalu in the northern part of the region.

 
These changes in litter sizes of the muskrats also reInforce a concept that

 
the population behavior of the gmuse and haress, and of rt-era 
 
phoAsants as well, .ei' reflect a ph      compambl. to a tigtening and 
 
relaxing of an advtrse influence operatlng with an apprwcli, tally 10-year

 
periodic ity. 
 
     (2) DP E             I"J1T       IFI P LA   N S1AYIOR. The iapor-

 
tance of density of population In conditioning rates of gain and loss of

 
vertebrates has been empýasised b- roaen& ±nsztightions,
It  s clear 
 
tihat,, Once poPUlations attain cýertain levels ) the net rates of
p-opulation 
 
gain tend to diminish and the loss rates to rise with incriased densities.

 
     of týe rate& of gain or loss n  lt     to density cono  
to mathr.t- 
 
cal curves, several agencies are, kno,ýwn to bec sufficiently influ~entiiil
on 
 
occasion to cause deviatIons from otherwise fairly predictable responses
of 
 
populations to their own densities, Includnge, In the uskrt,    tuan 
 
and episootics at irregular intervuls, stat low~ered rates of gain and 
 
accelerated rates of loss hawing no recognized connmection with any kncwn

 
 
-2ý