THE PHYLOGENETIC SERIES 395 



means then also that the rudimentary organ is an indication 

 of kinship with forms in which it exists in a functional condition. 

 8oi. If this principle is granted the kinship of the entire 

 organic world can be more or less completely established. 



802. The Phylogenetic Series. — A large part of the rocks 

 of the surface of the earth were formed by the deposits of mud, 

 sand and organic remains under water. Thus originated the 

 shales, sandstones, and limestones. While the rocks were 

 being deposited the bodies of animals and plants were frequently 

 buried and the resistant parts preserved as fossils. Of course, 

 the rocks which were formed first are now beneath those which 

 were deposited later. By the fossils and their relative posi- 

 tion in the rocks we have been able to learn something of the 

 character of the former inhabitants of the earth and of the 

 order in which they appeared. The geologist has been able to 

 reconstruct in considerable detail the more recent periods of 

 the earth's history, but the earlier chapters are difficult to 

 decipher. 



803. The Cambrian period is the earliest in which we find 

 any evidence of life. It was a period covering a vast extent of 

 time, and at its close most of the invertebrate phyla, if not all, 

 were already represented. The highest Molluscs, the Cephalo- 

 pods, were present, and also the aquatic Arthropods, the Crus- 

 tacea. Insects and spiders appeared in the next period, the 

 Silurian, and also the first Vertebrates, Fishes. In the Devonian 

 period Fishes were extremely abundant. The Amphibians and 

 Reptiles appeared in the Carboniferous period. Mammals 

 did not appear until the Jurassic period, and Birds came still 

 later, in the Triassic. So far as this evidence goes it shows that 

 the lower forms appeared first. With regard to Vertebrates, 

 more particularly, the order in which the classes appeared also 

 agrees with what one would expect according to the theory of 

 descent. 



804. The fact that Birds appeared after Mammals must not 



