242 FOOT-NOTES TO EVOLUTION. 



then a gyroceran, before taking on its own characters; 

 each gyroceran form went through these in the same 

 order up to its adult characters, but never reached the 

 close-coiled nautilian stage; while the orthoceran forms 

 remained in that stage of development all their lives. 

 These genera were all progressive, and are numerous 

 branches radiating from a common stock or radicle, the 

 primitive straight nautiloid. Thus different nautilian 

 forms may resemble each other closely, and yet be 

 actually more closely related to the radicle form than 

 they are to each other. Formerly such species were 

 grouped together in one genus, and called " representa- 

 tive " species ; now we know them to be merely morpho- 

 logical equivalents in different lines of descent. 



When the close coiled stage was reached the nautilian 

 shell had reached its limit, and could progress no fur- 

 ther, and at once some of the stock began to retrograde. 

 This is beautifully shown in the development of Lituites 

 (Plate V, Fig. 6), which goes through the orthoceran, 

 cyrtoceran, gyroceran, and nautilian stages, and as it 

 becomes adolescent leaves the close coil and reverts to 

 the orthoceran type. A number of other nautilian genera 

 acted in this way, giving rise to a number of aberrant 

 types. These reversionary nautiloids are confined to 

 the Paleozoic, and did not in any case become radicles of 

 later groups; they had run their course, exhausted the 

 possibilities of development, and died out without 

 descendants. But the old simple orthoceran type held 

 out until the Trias, and the unspecialized nautilian shell 

 endured until the present time, although now rapidly 

 Hearing extinction. 



The animals that are capable of giving the best proof 

 of evolution are the ammonoids. Somewhere back in 

 the Silurian an Orthoceras by acceleration of develop- 

 ment finally acquired a calcareous protoconch, or em- 



