Chap. XIV.] ORGANIC BEINGS. 231 



physiological importance ; why, in finding the relations 

 between one group and another, we summarily reject 

 analogical or adaptive characters, and yet use these 

 same characters within the limits of the same group. 

 We can clearly see how it is that all living and extinct 

 forms can be grouped together within a few great 

 classes ; 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 the affinities between the mem- 

 bers 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. 



Professor Hackel in his ' Generelle Morphologie ' and 

 in other works, has recently brought his great knowledge 

 and abilities to bear on what he calls phylogeny, or the 

 lines of descent of all organic beings. In drawing up 

 the several series he trusts chiefly to embryological 

 characters, but receives aid from homologous and rudi- 

 mentary organs, as well as from the successive periods 

 at which the various forms of life are believed to have 

 first appeared in our geological formations. He has thus 

 boldly made a great beginning, and shows us how 

 classification will in the future be treated. 



Moiyliolorjy . 



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 organisation. This resem- 

 blance is often expressed by the term " unity of type ; " 

 or by saying that the several parts and organs in the 

 different species of the class are homologous. The 



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