8 
INTRODUCTION. 
cal conditions; remembering their less specialised and complex structure, 
we can see that the influence of altered conditions might produce great 
variations in structure, in habits, in life history ; the pressure of com¬ 
petition would arise, supposing there were fewer checks ; (what checks 
there may have been is doubtful but both parasitic and predaceous 
insects, as well probably as insectivorous birds arose later and these are 
now the main checks); some, from feeding on decaying vegetable matter, 
might come to feed on decaying animal matter, with a consequent 
change of habits, of structure, of senses, possibly of life history ; others 
might find growing plants provided an ample supply of food and their 
descendants gradually get modified to suit these circumstances ; in time 
we can imagine some becoming predaceous, the descendants perhaps 
of insects that fed on dead insects ; we can still see the stages between 
land and aquatic insects, and it requires little imagination to picture 
the necessary gradations from an insect feeding on decaying leaves by 
a riverside, to one that entered the river water and found its food 
there. Given a plastic structure capable of modification, granted grow¬ 
ing competition and a free unoccupied field, one can readily see how, 
in earlier ages, the various groups may have arisen ; with the alter¬ 
ing conditions of successive geologic periods, with the evolution of 
higher plants and animals, with alterations of climate and natural con¬ 
ditions, one can realise how the diversity of forms of insect life would 
be evolved. That this has occurred with other forms of life one can 
read ; that the steps cannot be traced so clearly in insects is due to 
the imperfection of the geological record, insects being small, soft and 
not so fitted for preservation as are bones or shells. Granting that in 
previous ages this occurred, and seeing the present dominance of insect 
life on the earth and in fresh water, it is easy to see that the competition 
might be so severe that more and more complex structures, instincts 
and habits might be evolved leading steadily away from plasticity to 
more and more fixed and unalterable types ; the more primitive and 
simple insect feeding on decaying leaves, having simple biting mouth- 
parts, laying eggs in the ground, requiring no special colouring or pro¬ 
tective devices disappeared ; predaceous insects require more complex 
trophi ; quick flight necessitates better wings and a more consolidated 
thorax; protection from birds implies protective attitudes, colouring 
or form, and may require possibly the nocturnal habit, which implies 
