130 EVOLUTION AND GENETICS 



The next geological division, the early Paleozoic, records so 

 many forms of invertebrates that we are able to see in the fossils 

 many evidences of phylogenetic succession. However, this sudden 

 appearance of many forms deprives us of the opportunity to see 

 how and from what they arose. We are forced to begin in the 

 middle of the record, and by the steps which are clearly disclosed in 

 its more complete portion to judge as best we can what processes 

 took place in the periods which are forever closed to us. 



Through the remaining periods we are confronted by similar 

 difficulties. It seems that while hard parts are necessary for 

 effective fossilization, the development of such parts is character- 

 istic of specialized organisms, and not of those generalized species 

 which might be expected to give rise to specialized forms. The 

 record of detailed phylogeny is therefore confined to minor groups 

 of relatively high development. We are able to trace descent from 

 genus to genus in the ammonoids, for example, in the Devonian 

 and later periods, and in the related nautiloids, which probably 

 arose in the Cambrian. In the Ordovician, however, there are 

 fossils of armored fishes of whose ancestors we have no exact 

 record, and in the Devonian, we find evidences of amphibia, al- 

 though no intermediate form connecting them with the fishes is 

 known. 



However, the major succession is established even by these 

 abrupt transitions in the geological record. The lung fishes, able 

 to breathe air, are known to have occurred in the Silurian, prior to 

 the Amphibia, the Amphil)ia precede the wholly terrestrial verte- 

 brates, reptiles precede mammals and man is the last species of all 

 to appear. While a chapter is gone here and there, the entire 

 record, together with such details as are well preserved, is con- 

 vincing evidence of gradual succession. 



Succession Within Animal Phyla. Among the invertebrate phyla, 

 the status of individual groups also shows definite succession. 

 The Protozoa, now recognized as the most primitive existing ani- 

 mals, are too small and delicate to be readily fossilized. Some are 

 specialized, however, in the production of calcareous, siliceous, or 

 chitinous tests, and consequently are readily preserved (Fig. 70). 

 The Porifera, also very low in the systematic scale, are preserved 

 because of their hard spicules, and are found among the earliest 

 Paleozoic fossils in the Cambrian. The same is true of such Coelen- 

 terates as are preserved. Worms, because of their soft ])odies, are 



