379 
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 Mautilus pomptlius, 
shown on PI, i, can be easily compared with those of their supposed 
nearest congeneric shells, Barrandeoceras of the Silurian given on 
Pl. 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 
simiar 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 
