402 HISTORICAL GEOLOGY. 



In illustration : the vertebrated tails of the ancient Ganoids is one ex- 

 ample, since this feature is a characteristic of the young of living Ganoids, 

 and also of some other living fishes. The cartilaginous skeleton of the 

 ancient Ganoids is another embryonic feature. The stem of the ancient 

 Crinoids occurs in the young of the related Comatula. The Mastodon, as 

 regards its teeth, says Agassiz, and in some other points, is embryonic in its 

 relations to the Elephant. 



Paleontologists of skill derive a degree of prophetic power through tlie 

 aid of the canon. The shells of Ammonites have been shown by A. Hyatt to 

 afford an excellent illustration of the principle. Noting that the coiled shell 

 contained within it all the forms it had passed through from the embryo stage 

 to the adult, he proved by his studies of the shells of different genera that the 

 embryological succession corresponded in a general way with the geological 

 succession, and hence that the position in the geological scale of any new 

 species was approximately determinable from its form. It is obvious that 

 through the knowledge thus obtained stratigraphical doubts may often be 

 removed. Moreover, where direct paleontological observation has ascertained 

 in particular cases the steps of progress in the development of organs, as, for 

 example, those of the teeth in Mammals, the facts become a basis for further 

 use in the same direction. But decisions on such grounds have to be made 

 with great reserve ; since there were often, throughout paleontological his- 

 tory, retrograde steps in the various tribes of species, and, not unfrequently, 

 in some organs when the general progress was upward. Man stands at 

 the head of Mammals, and yet, as regards his teeth, he is below the Monkeys, 

 and related to the earliest Tertiary Mammals. 



By the methods which have been above described, great progress has 

 been made in arranging the rocks of the different continents in a chronologi- 

 cal series. North America has large blanks in the series which in Europe 

 are filled. In this and other ways the countries of the world are contribu- 

 ting to a general system of life history. 



Precautions in the use of fossils for correlation. — Precaution is required 

 for the following reasons : — 



1. The difference in species attending difference of conditions in climate, 

 soil, etc. In the same regions, during any era, the species of the land differ 

 from those of the waters ; those of fresh water from those of salt ; those of 

 the surface or shallow waters from those of deeper ; those of warm waters 

 from those of cold, whether at the surface or in the deep ocean where 

 oceanic currents make differences of temperature ; those of warm or dry 

 lands from those of cold or wet; those of clear open seas from those of 

 muddy waters or near muddy seashores ; those of rocky bottoms from those 

 of muddy ; etc. Hence, an ancient rock made in a clear sea, as a limestone, 

 Avill necessarily contain very different fossils from a rock that was made of 

 mud, although they were formed at the very same time, in the same waters, 

 and within a hundred miles of one another. Even a hundred yards may be 

 all that separates widely different groups of species. Again, a rock made 



