458 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
tL OR 
0 i. NOR 
Pe: 
pee R 
PA cs 
es ‘ 
= H 
ow 47% ' 
c 
° ; 
v 
rt 
wi 5 ' x 
® é 
w i 
& 
6 : 
a. < 1 
vy 1 
© p 
SA Ai 
oa : 
& G ' 
© 8r; 1 
= 3 
1 " H 
© 
a 
9, & 
Ficure 2.—The general iform of the rocking E 
response. A, The extremes of movement in = ‘ 
Dirphia eumedide as seen from in front, the » 1 ¢ 
wings having been removed. The two £ \ 
drawings represent the farthest points = i 
reached in a left and a right swing respec- 
tively. B, The pattern and timing of the lil ¢ S 
response in D. (Periphoba) hircia. Note 
the acceleration of the response toward its 
close. In this species the head alone is 12+. 4 
moved. : 
13 
14 
B. 
capacity of the species as a whole. In fact, it has been argued 
(Medawar, 1952) that, in most cases, the decline in reproductive po- 
tential with age is necessarily accompanied by a decrease in the 
selective forces acting on the individual. Exceptions to this general- 
ization have been recognized in the social animals, where group selec- 
tion can act to lengthen the postreproductive life of individuals whose 
