240 THE ORIGIN OF SPECIES 



however different they may be from their parents; aud I 

 believe that this element of descent is the hidden bond 

 of connection which naturalists have sought under the 

 term of the Natural System. On this idea of the natural 

 system being, in so far as it has been perfected, genea 

 logical in its arrangement, with the grades of difference 

 expressed by the terms genera, families, orders, etc., we 

 can understand the rules which we are compelled to fol- 

 low in our classification. We can understand why we 

 value certain resooiljlaiioes far more than othoi's; why 

 we use rudimentary and useless organs, or others ol tri- 

 fling physiological importance; why, in finding the rela- 

 tions between one group and another, we summarily re 

 ject 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 affin- 

 ities. We shall never, probably, disentangle the inextri- 

 cable web of the 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. 



Professor Haeckel, 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 char- 

 acters, but receives aid from homologous and rudimentary 

 organs, as well as from the successive periods at which 

 the various forms of life are believed to have first ap- 



