12 INTRODUCTION. 



standing, we should probably find this a matter of very great 

 difficulty. The moment, however, that they commenced to 

 go — or, in other words, to perform their own peculiar func- 

 tion — 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 /Mwciiow, there would be an equivalent dif- 

 ference 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 en- 

 tail 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 morpholc^ical 

 type, and they belong to the same sub-kingdom. They lead 

 different lives, however — they are placed under different con- 

 ditions — and they discharge different functions in the general 

 economy of Nature. They differ, therefore, physiologically ; 

 and, as every physiological difference impUes a corresponding 

 structural difference, they differ structurally as well. But 

 they differ structurally only because they differ physiologi- 

 cally, 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 dis- 

 solved in water. The butterfly is agrial, 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 corresponding 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 no distinct breathing-organs. It is only by the com- 

 bined effect of a number of these physiological differences, 

 taken collectively, that the lobster and the butterfly come ulti- 

 mately to be so strikingly distinct from one another 



It is now possible to comprehend fully the principles upon 



