CiiAi-. Xlll. ORGANIC BEINGS. 339 



connected with the early progenitors of the other and at that 

 time less diirercntiated vertebrate classes. Tlicre has been 

 less complete extinction of the forms of life which once con- 

 nected fishes with batrachians. There has been still less in 

 some other classes, as with the Crustacea, for here the most 

 wonderfully diverse forms are still linked together hy a long 

 and only partially-broken chain of allinities. Extinction has 

 only se})arated the groups : it has by no means made them ; 

 for, if every form which has ever lived on this earth were sud- 

 denly to reappear, tliough it would l)e cjuite impossi])le to give 

 (Iclinitions by which each group could be distinguished, still a 

 natural classification, or at least a natural arrangement, Avould 

 l)c j)ossible, AVe shall see this by turning to the diagram: 

 tlie letters, A to L, may represent eleven Silurian genera, some 

 of which have produced large groups of modified descendants, 

 Avjtli every link in each branch and sub-branch still alive ; and 

 the links not greater than those between the finest varieties. 

 In this case it would be quite im{)ossible to give definitions by 

 which the several members of the several groups could be dis- 

 tinguished 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, front A, Avould have something 

 in common. In a tree we can distinguisli this or that branch, 

 though at the actual fork the two unite and blend together. 

 AV'e could not, as I have said, define the several groups ; but 

 we could pick out t\'pes, 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 Ave should be driven to, if we were over to suc- 

 c(^ed in collecting all the foi-ms in any one class which have 

 lived througliout all time and sjiace. Assuredly we shall never 

 succeed in making so perfect a colh^ction : nevertheless, in cer- 

 tain classes, we arc tending toward this end; and Milne Ed- 

 wards has lately insisted, in an able paper, on the high impor- 

 tance of looking to types, whetlier or not we can separate and 

 define the groups to which such types belong. 



Finally, we have seen that natural selection, which results 

 from the struggle for existence, and which almost inevitably 

 leads to extinct ion and divergence of character in the descend- 

 ants from one dominant ]ia rent-species, explains that great and 

 universal feature in tlie atlinities of all organic beings, namely, 

 their suboiilination in group under group. We use the clc- 



