148 
smaller and more cylindrical in their whorls, 
smoother, and to lose their complicated foliated 
sutures. In extreme cases they became again 
perfectly straight cones, like the orthoceratitic 
radicals. So much alike are they, that it is 
quite common for those who are not students 
of this group to mistake the degraded Baculites 
for the radical Orthoceras. ‘This decrease in 
size, increasing smoothness, and uncoiling, is 
precisely parallel with the similar transforma- 
tions taking place during old age in the normal 
involute shells of the Jura, which, when old 
enough, also depart from the spiral, or tend to 
straighten out, and always lose their orna- 
ments, decrease in size, and so on.! 
The universal action of the surroundings, as 
we now know them, is certainly not exclusively 
favorable to the continuance of life, and may 
be wholly more or less unfavorable. It cer- 
tainly perpetually excites the animal to new 
and more powerful exertions, and, like per- 
petual friction, wears out its structures by the 
efforts which it obliges it to make for the sup- 
port of the structures in doing work. At first 
this leads to development, the supply being 
greater than the demand ; but sooner or later, 
and with unvarying certainty, the demand ex- 
ceeds the powers of supply, and old age sets 
in, either prematurely, or at the termination of 
the usual developmental periods. ‘The remark- 
able and at present unique example of the 
Ammonoidea places us in a position where we 
can see the same process taking place in the 
whole of a large group, with attendant phenom- 
ena similar in every respect to those which we 
have observed in individual shells of the same 
order. 
In numbers of species and genera, and in 
the complication of the internal structures 
and the production of the external ornaments 
on the shells, the order reaches what appears 
to be the highest stage of development 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 process in their respective 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 compari- 
son 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 
1 We are aware of the existence of evidence that Ammonites 
of the normal form, the types of which we have seen, have been 
described from the lower tertiaries: but there are still doubts 
about the reputed age of the formations; and, in any case, they 
only tend to confirm the general trend of the facts. 
SCIENCE. 
enable us to explain, in some measure, the © 
equivalent phenomena of the life of the group; 
and we are unavoidably led to entertain the ex- 
pectation that it does explain it. ‘This expec- 
tation was actually formulated as a probable 
law for the whole animal kingdom by Haeckel 
in the same year (1866) as the author first 
published on the Tetrabranchiata. We are 
therefore able to quote this leader in science 
in support of our weaker knowledge; and also 
a pupil of his, Wurtemburger, who has an- 
nounced the same results attained by researches 
on the Ammonites of the Jura, but, naturally 
perhaps, omitted to recognize any one but his 
honored master. 
The evidence is very strong, that there is a 
limit to the progressive complications which 
may take place in any type, beyond which it 
can only proceed by reversing the process, and 
retrograding. 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 Nauti- 
loidea and Ammonoidea these are the radical 
or generator types. We have no case yet of 
a highly complicated, specialized type, with a 
long line of descendants traceable to it as the 
radical, except the retrogressive: but all our 
examples of radicals are taken from lower, 
simpler forms; and these radical types are 
longer-lived, more persistent, and less change- 
able in time, than their descendants. 
We find the radicals of the Nautiloidea liv- 
ing throughout the paleozoic, and perpetually 
evolving new types in all directions ; then this 
process ceases, and the primary radicals them- 
selves 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 radi- 
cals, and generate series having more involute 
shells. These, in turn, as secondary radicals, 
exhibit very decidedly a greater chronological 
distribution than their descendant inyvolute 
forms, persisting, even to the present day, in 
Nautilus umbilicatus. 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 Nau- 
tiloidea, even as far back as the Cambrian. 
These secondary radicals, greatly modified but 
still carrying in their simpler organizations and 
mode of coiling the possibilities of a number — 
of new series, existed by the side of the ex- — 
piring degraded forms of the cretaceous. Ee 
This is the essential element of difference — 
between the life of the whole order and that of © 
