GHOLOGICAL SUCCESSION OF ANIMALS. 669 
sea-urchins and of ammonites, the older being of simpler, 
more generalized forms, and the later with a greater 
specialization or elaboration of the different, especially ex- 
ternal, hard parts of the body. 
When we ascend to the Amphibians, the reptiles and the 
mammals, we shall find that there has been an elaboration 
or working out into great detail, of the parts most used by 
the animal, this differentiation being more and more marked 
as we approach the present time; and this has been in ac- 
cord with the building up of the continental masses, and 
the differentiation or specialization of the surface of the 
different continents into plains, plateaus, highlands, and 
mountain ranges, with their different climatic features, 
and the dividing up of the waters into mediterranean 
seas, friths, fiords, rivers, and lakes. Thus the extinction 
of successive faune all over the globe has been followed by 
the appearance of new sets of animals, each assemblage be- 
ing adapted to the new and improved condition of things. 
Having seen that the earlier forms of life were of a sim- 
pler form, though often combining the features of diverse 
classes and orders of animals which appeared afterward, so 
that Agassiz called them, in some cases, prophetic types, 
combining as they did characters which have been trans 
mitted to two or more later groups, and these specially elab- 
orated, so that such generalized or prophetic types serve as 
points of departure from which severai series of forms have 
arisen—having traced the law or principle underlying the 
geological succession of animals, we may inquire whether 
this has been paralleled by the development of any one of 
the members of a group. That this is the case has been 
proved by Hyatt, who shows that the development of the 
individual Ammonite is paralleled by that of the geological 
succession of the members of the order to which it belongs. 
Stalked Crinoids were the style in Paleozoic ages, while free 
Crinoids are more abundant at the present day; and we 
have seen that in the individual development of the existing 
Antedon, the young is stalked at first, afterward becoming 
free. The young, bony fish has at first a cartilaginous 
skeleton and a heterocercal tail, these being characteristics 
