i84 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



diminishing in the Devonian and Carboniferous and 

 ultimately ceasing their existence altogether in the 

 Trias. The fourth ceases in the Cretaceous, and the 

 fifth alone survives in the Tertiaries and is still living 

 in the Nautilus umbilicatus and pompilus, and two other 

 species. 



"Wherever found, the young of shells of the fifth 

 kind are at first orthoceran or cyrtoceran like the first 

 and second kind, then gyroceran in curvature like the 

 third class, and then they become more or less rapidly 

 nautilian like the fourth class in succeeding stages. In 

 Silurian, Devonian, and Carboniferous forms this suc- 

 cession is so marked that about all of the young shells 

 of the fifth class may be described as palingenetic, 

 that is as cyrtoceran, gyroceran, nautilian, and then 

 involute-nautilian in their individual or ontogenetic 

 development. In the Trias, Jura, Cretaceous, Ter- 

 tiary, and present, as the fifth class increases in num- 

 bers, there is a decided tendency to shorten and su- 

 persede the gyroceran or third stage and introduce the 

 fourth kind or the tendency to spread by growth in- 

 wards, at earlier stages. 



"The characteristics of^the sutures are correlative 

 with these stages of development, and it may be said 

 in a general way, that all other characteristics corre- 

 late more or less when studied comparatively in dif- 

 ferent series with the differences in the curvature and 

 coiling of the whorls. • The curvature and amount of 

 involution is therefore the most important single char- 

 acteristic of the nautiloids,' so far as the comparative 

 study of change by evolution is concerned; whether 

 the whole order be considered statistically as above, 

 i. e. with reference to the existence or non-existence 

 of certain forms orthoceran, cyrtoceran, etc., or gen- 



