230 AFFINITIES CONNECTING [Chap. XI 7. 



and thus give a general idea of the value of the differ- 

 ences betvs'een them. This is what we should be driven 

 to, if we were ever to succeed in collecting all the forms 

 in any one class which have lived throughout all time 

 and space. Assuredly we shall never succeed in mak- 

 ing so perfect a collection: nevertheless, in certain 

 classes, we are tending towards this end; and Milne 

 Edwards has lately insisted, in an able paper, on the high 

 importance of looking to types, whether or not we can 

 separate and define the groups to which such types be- 

 long. 



Finally, we have seen that natural selection, which 

 follows from the struggle for existence, and which al- 

 most inevitably leads to extinction and divergence of 

 character in the descendants from anyone parent-species, 

 explains that great and universal feature in the affinities 

 of all organic beings, namely, their subordination in 

 group under group. We use the element of descent in 

 classing the individuals of both sexes and of all ages 

 under one species, although they may have but few char- 

 acters in common; we use descent in classing acknowl- 

 edged varieties, however different they may be from 

 their parents; and I believe that this element of de- 

 scent is the hidden bond of connection which natur- 

 alists have sought under the term of the ISTatural System. 

 On this idea of the natural system being, in so far as it 

 has been perfected, genealogical in its arrangement, with 

 the grades of difference expressed by the terms genera, 

 families, orders, &c., we can understand the rules which 

 we are compelled to follow in our classification. We 

 can understand why we value certain resemblances far 

 more than others; why we use rudimentary and useless 

 organs, or others of trifling physiological importance; 



