12 INTRODUCTION. 
form their own peculiar function—we should soon see that 
some would be different to the others. Some, for instance, 
would strike the hours, and these would have to be laid aside 
in a group by themselves. And we should further discover 
that in accordance with the difference in the /wzctzon, there 
would be an equivalent difference in the structure, of these two 
groups. The striking watches would be formed upon the same 
fundamental type as those which did not strike; but, in addition 
to the broad and general details of structure in which all were 
the same, the striking watches would have a special apparatus or 
structure fitted for striking the hours, The non-striking watches 
would be destitute of this apparatus, so that the physiological 
or functional difference between the two groups would thus 
entail a corresponding difference in structure. 
It is just the same with animals. If we take a lobster, a 
butterfly, a scorpion, and a spider, we find that, dissimilar as 
they are in external appearance, they are all constructed upon 
the same fundamental plan. They agree in morphological type, 
and they belong to the samesub-kingdom. They lead different 
lives, however—they are placed under different conditions—and 
they discharge different functions in the general economy of 
nature. They differ, therefore, physiologically ; and as every 
physiological difference implies a corresponding structural dif- 
ference, they differ structurally as well. But they differ struc- 
turally only decause they differ physiologically, and in all the 
really essential details of their structure they are the same. 
The lobster is aquatic in its habits, and has therefore gills, or 
organs adapted for breathing air dissolved in water. The but- 
terfly is aerial, and has respiratory organs adapted for breathing 
air directly, and not through the medium of water. They differ, 
then, physiologically, and therefore, necessarily, in the corre- 
sponding structure. Both, however, have distinct organs set 
apart and dedicated to the function of respiration. This is an 
essential and fundamental point in their structure, and in this 
they both agree with one another and differ from a large number 
of animals in which there are wo distinct breathing-organs. It is 
only by the combined effect of a number of these physiological 
differences, taken collectively, that the lobster and the butterfly 
come ultimately to be so strikingly distinct from one another. 
It is now possible to comprehend fully the principles upon 
which a naturalist proceeds in framing a classification of the 
