MUTUAL AFFINITIES OF ORGANIC BEINGS 389 



those between existing varieties. In this case it would be quite im- 

 possible to give definitions by which the several members of the 

 several groups could be distinguished from their more immediate 

 parents and descendants. Yet the arrangement in the diagram 

 would still hold good and would be natural; for, on the principle 

 of inheritance, all the forms descended, for instance, from A, 

 would have something in common. In a tree we can distinguish 

 this or that branch, though at the actual fork the two unite and 

 blend together. We could not, as I have said, define the several 

 groups; but we could pick out types, or forms, representing most 

 of the characters of each group, whether large or small, and thus 

 give a general idea of the value of the differences between 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 through- 

 out all time and space. Assuredly we shall never succeed in making 

 so perfect a collection: nevertheless, in certain classes, we are 

 tending toward 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 belong. 



Finally, we have seen that natural selection, which follows 

 from the struggle for existence, and which almost inevitably leads 

 to extinction and divergence of character in the descendants from 

 any one 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 characters in common; we use 

 descent in classing acknowledged varieties, however different 

 they may be from their parents; and 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 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; why, in finding the relations be- 

 tween 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 



