Lehmann i —Variations in Form and Size of Cyclops. 291 
cific limits for either form!. These characters' must then he con¬ 
sidered as unreliable for specific distinction between C. brevis- 
pinosus and C. americanus. Of course if other features of suf¬ 
ficient importance for distinction prove reliable, the two species 
may stand. 
The great variation found in the specimens obtained from 
pools subject to marked changes, and the greater uniformity 
found among those living under comparatively uniform con¬ 
ditions, indicates that there is a tendency toward at least vari¬ 
etal distinctions through constant peculiar environment. The 
uniformity in tbe armature of the fourth swimming feet of 
specimens living in large bodies of water illustrates this. This 
armature is, however, not peculiar to those forms. It is of in¬ 
terest also to note that of the specimens obtained from large 
bodies of water, the greatest number of any one length or 
width of furca are of a measurement near an average of the 
limits of variation. 
The dividing lines between species are very difficult to con¬ 
struct, but it is certain that approximate limits of variation in 
variable features, as dependent on environment and habits of 
the animal, are of much determinative value; and a careful 
study of these would probably result in a re-classification of 
many species and varieties. 
I am greatly indebted to Professor Marsh for material, liter¬ 
ature, and kindly assistance in the preparation of this paper. 
