RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 297 



to have been coniined mainly to specific variation, and in many cases 

 this also was slight. These remarks apply equally to the gasteropods 

 and lamellibranchiates. Furthermore, the land and palustral pulmo- 

 nale mollusks which were contemporary with those just mentioned 

 seem as a whole to have been subject t> little if any greater degree of 

 differential evolution than were the others. It is true that progressive 

 evolution in the case of all these mollusks was also very slight, but 

 that does not explain the cause of the slight differential evolution. 



During the time that all those fresh water and land mollusks were 

 so slightly affected by evolutional change marine mollusks were not 

 only extremely differentiated, but many genera and some families suc- 

 cessively became extinct and many others were introduced. During 

 that time also some of the most important advances were made in both 

 progressive and differential evolution of animal ami vegetable forms 

 that have ever occurred upon the earth. So far as is now known all 

 exogenous plants began their existence since those mollusks began 

 theirs, and the earlier ones mentioned were contemporaneous with the 

 most tlourishing period of the dinosaurs. That great reptilian sub- 

 class passed its climax of development and became extinct, and yet 

 those mollusks were meantime but little changed. 



An example of extreme differential evolution is afforded by the Tri- 

 lobites, which in the early geological ages became greatly differenti- 

 ated, but from the time the order became established to that of its ex- 

 tinction there was comparatively little advancement in biological rank. 

 A somewhat similar example is afforded by the dinosaurs. While 

 their rank among reptiles was the highest the difference in average 

 rank between the earliest and latest known forms belonging to that 

 subclass is comparatively small and little, if any, in favor of the latter 

 forms. The Mammalia afford a notable example of both progressive 

 and differential evolution, ranging in time from the early Tertiary and 

 in rank ending with man. 



(6) The succession of gradual mutations in the development of the Leading classi- 

 ficatory features which characterize certain groups of fossil tonus was not neces- 

 sarily concurrent with consecutive portions of time. 



For example, the mutations of the flexures of the dental sac which 

 produced the various structural features of the teeth by which the dif- 

 ferent groups ofthe mammalia were characterized, or those of the mantle 

 in the production of the lobes and saddles ofthe septa of chambered 

 cephalopods, did not in either case occur along a single line of progres- 

 sive evolution, but along numerous differential lines coincident with each 

 of which the rate ofbiologieal progress was different from that of others. 

 Therefore advanced stages of progress must necessarily have been 

 reached on certain of those lines contemporaneously with much retarded 

 stages on others, and similar stages of progress were reached at more 

 or less widely separated intervals of time. 



This statement concerning 1 the dental features of the Mammalia and 



