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the shells, the order reaches what appears to be the acme of evolu- 
tion in the Jura; then retrogression begins, and, steadily gaining, 
finally affects all forms of the type, and it becomes extinct. Smaller 
series of the Ammonoidea and Nautiloidea go through the same pro- 
cess within their more restricted time-limits, and in the same way, 
but can be compared with the individual much more accurately and 
closely. It is evident, then, that the comparison of the life of an 
individual with that of its immediate series or group reaches a high 
degree of exactitude, and that the observed phenomena of the life 
of an individual should enable us to explain, in some measure, the 
equivalent phenomena of the life of the group; and we are unavoid- 
ably led to entertain the expectation that it does explain it. 
The evidence is very strong that there is a limit to the progres- 
sive complications which may take place in any type, beyond 
which it can only proceed by reversing the process, and retrograd- 
-ing. At the same time, however, the evidence is equally strong 
that there are such things as types which remain comparatively 
simple, or do not progress to the same degree as others of their 
own group. Among Nautiloidea and Ammonoidea these are the 
radical or generator types. No case has yet been found of a highly 
complicated, specialized type, with a long line of descendants tracea- 
ble to it as the radical, except the progressive: and all our examples 
of radicals are taken from lower, simpler forms; and these radical 
types are longer-lived, more persistent and less changeable in time 
than their descendants. 
We find the radicals of the Nautiloidea living throughout the 
Paleozoic, and perpetually evolving new types in all directions; 
then this process ceases, and the primary radicals themselves die 
out. But they leave shells, which are in that stage of progression 
which I have called the nautilian. These, the more direct descend- 
ants of the radicals, become secondary radicals and generate series 
having more involute shells. These, in turn, as secondary radicals, 
exhibit a greater chronological distribution than their descendant 
involute forms. The same story may be told of the Ammonoidea, 
but substituting at once the close-coiled shell (the secondary radicals) 
for the primary radicals of the Nautiloidea, even as far back as the 
Devonian. 
This is the essential element of difference between the life of the 
whole order and that of the individual. One can accurately com- 
pare the rise and fall of the individual and its cycle of transforma- 
