540 The American Naturalist. [June, : 
(2.) As a rule, not without exceptions, the legs vary together 
in a given direction ; that is, if in a certain individual the 
first pair are longer than the first pair of another individual, 
the other legs of the former are likely to be longer than the 
corresponding legs of the latter. The hind legs are the most 
irregular in their variation. 
(3.) There is a gradual increase in size of body and length 
of legs in the individuals of a species from the north, south- 
ward at least as far as latitude 37° in the Mississippi Valley 
near which region the sub-family Phalangiine appears. to 
attain its maximum development. 
(4.) The proportionate increase in the length of the legs to 
the southward appears to be greater than that of the body. 
These last two propositions are true of all the harvest- 
spiders having an extended northern and southern distribution 
studied with reference to their variations, viz; Liobunum 
vittatum, L. ventricosum, L. politum and L. longipes. Probably 
the most important factor in determining this increase in size 
is to be found in the climatic conditions which permit a 
longer period of growth and feeding at the south than in the 
north. The arachnids undergo no definite series of molts, 
simply casting their skins as increase in size requires. Such 
climatic conditions, combined presumably with a more 
abundant food supply, may tend, apparently, to increase the 
size of these harvest-spiders by at least three methods: 
(a.) By the direct effect of a long warm period of growth 
with abundant food upon the individual before attaining its 
full size. 
(b.) By the effect of a long period of development of the eggs 
in the ovaries of the female. 
(c.) By the action of the eliminating principle in natural selec- 
tion in fostering those individuals which attain the maximum 
size compatible with their environment. 
This increase in size of individual may possibly be of bene 
fit to the species (1) by enabling it to prey upon larger insects, 
thus increasing the source and variety of its food-supply ; (2) 
by enabling it better to elude predaceous enemies, or (3) to 
catch fleeing prey. 
