Hyatt.] 168 [Dec. 3, 



Darwinian theory, and their quick or sudden production, according 

 to the law of acceleration, as explained by Cope and the writer, 

 and subsequently by Mivart. The gaps between forms or species, 

 may be largely explained by the latter mode of development, if the 

 necessary care is taken to study the earlier stages, which should show 

 the close genetic connection of the distinct adult forms, and explain 

 thereby the absence of the intermediate varieties. By carefully ob- 

 serving these principles, it is possible to trace the entire family of the 

 Arietidse to one variety of one species, the smooth variety of Psiloce- 

 ras planorbis. 



The species are evolved from this single form in series of various 

 kinds, some forming lines passing up through the various subsequent 

 formations from the planorbisbed, giving origin in their turn to other 

 series, and some remaining single. 



Thus the whole picture is comparable to a genealogical tree, the 

 trunk represented by the smooth psilonotus, which originates in the 

 Triassic formation, and giving rise on either side to a fan-shaped 

 array of branches, each branch representing a series of forms or 

 species, and quite often having smaller branches of its own. In each 

 case the point of origin of the branches or series is near the point of 

 origin of the branch from which they spring, whence the fan-shaped 

 arrangement alluded to above. Farther, each series perfects or car- 

 ries a certain series of characteristics common to itself, and a certain 

 series common to the whole family. 



The first distinguish it as a genus or group, and the latter are the 

 parallel or mimetic characteristics which are regularly produced in 

 each individual, and each series, according to the place of the indi- 

 vidual in the series. 



Thus psilonotus is smooth throughout life, Arnioceras is smooth 

 for a certain period in the young, then adds ribs, and a keel, then 

 channels. The adult Conybeari adds tubercles on the ribs, which 

 appear in a young stage of Coroniceras, a later appearing series, and 

 then in the last number of its own series becomes more involute; this 

 greater degree of involution appearing as a young characteristic in 

 the last series of the family Asteroceras. 



The old age changes observable in psilonotus are very slight, they 

 become greater and more distinct in Caloceras, and succeeding series; 

 finally in the last members of the series of Coroniceras they seriously 

 affect the entire form of the adult, and in Asteroceras the adult of 



