10 IXTRODUCTION. 



and what splendid generalizations can be drawn from what at 

 first sight would appear to be the most fragmentary evidence. 



This imaginary illustration exactly expresses the points 

 which are to be regarded as essential and fundamental in clas- 

 sifying and arranging animals. We have to look, namely, 

 firstly^ to the plan upon which each animal is constructed ; 

 secondly, to the manner in which it discharges its vital func- 

 tions. These are the two points of view from which every 

 organism may be regarded — in their nature quite distinct, and 

 indeed sometimes apparently opposite. From the one point 

 of view we have to look solely to the laws, form, and arrange- 

 ment of the structures of the organism. This constitutes what 

 is technically called " Morphology," or the science of form 

 (from the Greek words, morpM, form ; and logos, a discourse). 

 From the^second point of view, we are concerned simply with 

 the functions discharged by the different parts of the organ- 

 ism, and this constitutes what is known as " Physiology." It 

 is most important to remember that there are no other points 

 in which it is possible for one animal to differ from another. 

 If two animals are different, they must differ in one or other 

 or in both of these points. Either they differ morphologically, 

 in being constructed upon altogether different plans ; or they 

 differ physiologically, in performing a different amount of 

 vital work in a different manner, and with different instru- 

 ments ; or they differ both morphologically and physiologi- 

 cally. Philosophical classification, therefore, insomuch as it 

 depends entirely upon a due appreciation of what are the 

 real differences between different animals, is nothing more 

 than an attempt to express formally the facts and laws of Mor- 

 phology and Physiology. 



Examining next into the nature and extent of the morpho- 

 logical or structural differences between different animals, we 

 find that these are much less and much fewer than might have 

 been thought. By one not previously acquainted with the 

 subject, it might readily be supposed that every kind of ani- 

 mal was constructed upon a type or plan peculiar to itself and 

 not shared by any other. We should certainly suppose, for 

 example, that animals so different as a lobster and a butterfly 

 were built upon different types or plans of structure. When 

 we come, however, to examine the question, we find that this 

 is not the case. The lobster and the butterfly are constructed 

 upon the same structural plan or morphological type. What 

 is still more remarkable, we find that all known animals, in 

 spite of their immense differences in external appearance, are 



