PALEOZOIC ROCKS AND ERA. 297 



afterward developed the more typical forms. To illus- 

 trate : The first fishes were not typical fishes, but con- 

 necting links between fish and amphibian, and from this 

 intermediate form, as from a trunk, true fishes and 

 amphibians were afterward separated and developed as 

 branches. Such intermediate forms we shall hereafter 

 call generalized forms, and the more typical forms into 

 which they seem to be afterward developed, specialized 

 forms. We shall find many illustrations of this law as 

 we proceed. 



Apparent Suddenness of the Appearance of 

 Fishes. At a certain time fishes seem suddenly to appear, 

 as if they came without progenitors. But we must re- 

 member that the very lowest forms of fishes have neither 

 bony skeleton nor scales, and their remains are not likely 

 to be preserved. We may yet find evidences of such far 

 down in the Silurian. Nevertheless, there can be little 

 doubt that conditions were favorable for the development 

 of fishes about the beginning of the Devonian, and there- 

 fore the steps of development were exceptionally rapid at 

 that time. 



SECTION IV. CARBONIFEROUS SYSTEM. AGE OF ACRO- 

 GENS AND AMPHIBIANS. 



Subdivisions. The Carboniferous age is subdivided 

 into three periods : 1. Sub-carboniferous ; 2. Carbonifer- 

 ous proper, or coal-measures ; 3. Permian. The first may 

 be regarded as the preparation, the second the culmina- 

 tion, and the third the transition to the Mesozoic. The 

 whole carboniferous strata in Nova Scotia is 16,000 feet 

 thick, in Wales 14,000 feet, in Pennsylvania 9,000 feet. 



The sub-carboniferous strata are mostly limestones ; 

 those of the coal-measures mostly, though not wholly, 

 sands and clays. The sub-carboniferous are marine de- 

 posits, the coal-measures mainly fresh-water deposits, 



