424 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
are more complex or generalized, becoming more or less ornamented 
with spines and various kinds of markings; or in the case of 
the ammonites, the septa, at first simple, become divided into 
folds, and these later become more complex by being beautifully 
frilled, showing a high degree of specialization. Then the vitality 
of the over-ornamented organism wanes; it falls into a decline, and 
old age characteristics develop, such as the effacement of tubercles, 
folds, and frills, and there is a return, in the descendants of the 
specialized forms, to the simplicity of form and lack of ornamenta¬ 
tions of the primitive ancestors of the group. In other words, there 
is in certain groups, as in the individual, a period of infancy? 
youth, maturity, and decline or senility, and in certain classes this 
is followed by the death or extinction of the class. Paleontology 
shows us that the type arises, becomes strong and successful in 
competition with others, then, as if overloaded and enfeebled by its 
luxurious growth, begins to decaj^ and finally is driven to the wall 
by the incoming of more vigorous, aggressive, and highly specialized 
types. Such has been the case with the classes of graptolites, 
trilobites, and ammonites. The same law obtains in the history of 
human races. Gibbon traced the rise, culmination, and decline of 
the Roman empire. Savage races fade away at the approach of 
those more civilized, or if they survive, show traces of decay and 
decrepitude. 
Now all these facts and laws were for many years a favorite study 
with Professor Hyatt, and he worked them out with great detail 
and thoroughness in his various papers on cephalopod molluscs. 
He traced the law of acceleration in some characters, and of retarda¬ 
tion in others, and showed that these laws were largely conditioned 
on changes in the environment, on competition, on use and disuse. 
He also extended and carried out the idea of Darwin and others 
that the evolution of species and genera was more rapid in the early 
geological periods than now. He emphasized the view that a type 
in beginning its career, on migrating into unoccupied regions, 
under new conditions of life, and free from comjDetition, varies, 
greatly accelerates its development, and thus gives rise to new 
groups. He worked this out for ammonites and in a specially 
striking way for the Steinheim tertiary shells, and was engaged on 
the same problem in the case of the existing land shells of the 
Hawaiian islands. Indeed, he had planned to go to these islands 
