390 ON THE ORIGIN OF SPECIES 



physiological importance ; why, in comparing* one group 

 with a distinct group, we summarily reject analogical 

 or adaptive characters, and yet use these same char- 

 acters within the limits of the. same group. We can 

 clearly see how it is that all living and extinct forms 

 can be grouped together in one great system ; and how 

 the several members of each class are connected together 

 by the most complex and radiating lines of affinities. 

 We shall never, probably, disentangle the inextricable 

 web of affinities between the members of any one class ; 

 but when we have a distinct object in view, and do not 

 look to some unknown plan of creation, we may hope 

 to make sure but slow progress. 



Morphology. — We have seen that the members of 

 the same class, independently of their habits of life, 

 resemble each other in the general plan of their organ- 

 isation. This resemblance is often expressed by the 

 term c unity of type ; ' or by saying that the several 

 parts and organs in the different species of the class 

 are homologous. The whole subject is included under 

 the general name of Morphology. This is the most 

 interesting department of natural history, and may be 

 said to be its very soul. What can be more curious 

 than that the hand of a man, formed for grasping, that 

 of a mole for digging, the leg of the horse, the paddle 

 of the porpoise, and the wing of the bat, should all be 

 constructed on the same pattern, and should include 

 similar bones, in the same relative positions ? Geoffroy 

 St. Hilaire has insisted strongly on the high importance 

 of relative connection in homologous organs : the parts 

 may change to almost any extent in form and size, and 

 yet they always remain connected together in the same 

 order. We never find, for instance, the bones of the 

 arm and forearm, or of the thigh and leg, transposed. 

 Hence the same names can be given to the homologous 

 bones in widely different animals. We see the same 

 great law in the construction of the mouths of insects : 

 what can be more different than the immensely long 

 spiral proboscis of a sphinx-moth, the curious folded 



