116 INSTINCTIVE BEHAVIOUR 
us to the conclusion that its evolution runs parallel with the 
evolution of animal structure. This is perhaps best seen in 
the case of those insects in which typical instinctive acts are 
performed by larvee of wholly different form and structure, 
though they are stages in the development of the same species. 
This is exemplified in the cases of Sitaris, Argyromeba, and 
Lewopsis which have, been briefly described. It is probable 
that in all cases of instinctive action natural selection has been 
a co-operating factor. Without going so far as to assert with 
Professor Weismann the “all-sufficiency of natural selection,” 
we may echo the words of Professor Groos,* and say : “ Never- 
theless, we know no principle except that of selection, and we 
must go as far as that will take us. Absolute knowledge of 
‘such phenomena is unattainable.” And in this conclusion we 
have the support of Dr. Peckham, who says,f “ We have found 
them [the instinctive acts of solitary wasps] in all stages of 
their development, and are convinced that they have passed 
through many degrees from the simple to the complex, by the 
action of natural selection. Indeed, we find in them beautiful 
examples of the survival of the fittest.” 
* «The Play of Animals,”’ p. 64. 
t “Solitary Wasps,” p. 236. 
