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tions with that of any of the single series or branches of the same 

 stock which become highly specialized and then degenerate ; but, 

 when an attempt to go farther is made, similar difficulties arise to 

 those encountered in tracing the progress of types and orders. The 

 radical and persistent types are still present, and teach us that, as 

 long as they exist sufficiently unchanged, new types are a possibility. 

 I have traced a number of these in the two orders, and have 

 found that they change and became more complicated, and that 

 probably a purely persistent or entirely unprogressive type does not 

 exist among the fossil Cephalopoda. 



The most celebrated example of unchanging persistency has been, 

 and is now supposed to be, the modern Nautilus. The similarities 

 of this shell to some of the Silurian coiled forms — which have 

 caused Barrande and others to suppose that it might be transferred 

 to the same fauna without creating confusion — belong to the cate- 

 gory known to the naturalist as representative. It is similar in 

 form, and even in structure, in the adults, but has young with en- 

 tirely distinct earlier stages of development, and belongs to dis- 

 tinct genetic series. The young of the existing Nautilus pompilius , 

 shown on PI. i, can be easily compared with those of their supposed 

 nearest congeneric shells, Barrandeoceras of the Silurian given on 

 PI. v, Figs. 6-10. 



Comparative invariability or persistency is common to all 

 radicals ; and they force us to recognize the fact that the orders 

 could have produced new series, as long as they were present, 

 if it had not been for the direct unfavorable action of the physical 

 changes which took place, so far as we now know, over the 

 whole earth. Thus, in making comparisons between the life of 

 the individual and the life of the group, one cannot say that the 

 causes which produced old age and those which produced retrogres- 

 sive types were identical: it can only be said that they produced 

 similar effects in changing the structures of the individual and of 

 the progressive types, and were therefore unfavorable to the farther 

 development and complication of these types. In their effects they 

 were certainly similar ; but in themselves they might have been, 

 and probably were, quite different, agreeing only in belonging to that 

 class of causes usually described as pathological, or those whose 

 nature can be generally summed up as essentially unfavorable to the 

 progress, and even to the existence, of the organism. 



In order to understand the meaning of these evidently degraded 



