10 
INTRODUCTION. 
universal occurrence on land and in fresh water, the extraordinary 
variety in habits, food and ways of life, as compared with any other 
group or with all groups together, we can see that in no other class in 
the animal world is competition so keen, are instincts and habits so 
fixed, is the whole of life for each species so unalterable and delicate. 
Insects have lived, have dominated the earth, have become what 
we see them by carrying to an extreme the principle of adaptation to 
circumstances, of making the most of natural conditions ; man has 
become what he is, because he has carried to an extreme the principle of 
adapting natural conditions to himself while only adapting himself to 
them to a limited extent ; the two classes dominate the land, and when 
man cannot alter the conditions to make life permanently bearable, 
insects can adapt themselves and do. But in the process man has 
developed one form of mentality implied in the terms free-will, choice, 
volition, while insects have become perfect mechanical structures 
reacting in a definite way to natural forces and stimuli, their lives ruled 
by fixed and most perfect “ instincts.” 
It is not my intention to give the impression that instincts are 
absolutely fixed but only that they are fixed as compared with the plasti¬ 
city of earlier insects and as compared, say, with man. There is a 
certain latitude still, more in some groups than in others, but even in 
them not much and in the most specialised probably very little. I 
imagine that such simple forms as Machilis are fixed in their simple 
habits as compared with a Sphegid fixed in complex habits, but to both 
there is a certain small latitude within which they can still alter. The 
instincts of a polyphagous caterpillar such as Chloridea obsoleta are pro¬ 
bably much less fixed and specialised than are the instincts of the 
caterpillar of Scirpophaga aurif.ua , for instance, and in each case possibly 
their degree of specialisation, low or high, makes for success, success 
being purely the ability to get food and lay eggs freely. Some are- 
successful because they are fixed in delicate mechanical instincts, notably 
the insect-stinging wasps ; others are successful because they can adapt 
themselves still to a limited variation of circumstances, such as food, 
temperature, etc., and they are still to some extent plastic. But it is a 
very limited plasticity, little akin to the plasticity of the earlier forms 
from wdiich our present insect life has arisen. 
Classification.— When insects were first studied in some detail, 
