Io INTRODUCTION, 
manner in which it discharges its vital functions. These are 
the two points of view from which every organism may be re- 
garded—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 arrangement of the structures 
of the organism. This constitutes what is technically called 
“Morphology,” or the science*of form (from the Greek words, 
morphé, form; and /ogos, a discourse). From the second point 
of view, we are concerned simply with the /wzctions discharged 
by the different parts of the organism, 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 sorphologically, in being constructed upon altoge- 
ther different plans; or they differ physéologically, in performing 
a different amount of vital work in a different manner, and with 
different instruments ; or they differ both morphologically and 
physiologically. Philosophical classification, therefore, inso- 
much 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 
Morphology 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 sub- 
ject, it might readily be supposed that every kind of animal was 
constructed upon atype 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 struc- 
tural plan or morphological type. What is still more remark- 
able, we find that a// known animals, in spite of their immense 
differences in external appearance, are really constructed upon 
no more than some half-dozen primary plans of structure or 
morphological types. These types are all different from one 
another, but there is no animal yet known to us, living or ex- 
tinct, which cannot be referred to one or other of these six 
