ON THE ORIGIN OF GENERA. 51 



8. It is well known that the Cephalopoda form a number of 

 series of remarkable regularity, the advance being, in the first 

 place, in the complication of the folds of the external margins of 

 the septa, and, in the second place, in the degree of involution of 

 one or both extremities of the shell to the spiral ; third, in the 

 position of the siphon. 



Alpheus Hyatt, in an important essay on this subject,* 

 points out that the less complex forms are in many cases iden- 

 tical with the undeveloped conditions of the more complex. 

 He says : ^^ There is a direct connection between the position of 

 a shell, in the completed cycle of the life of this order, and its 

 own development. Those shells occujoying the extremes of the 

 cycle" (in time), ^Hhe polar forms, being more embryonic than 

 the intermediate forms, f The first epoch of the order is 

 especially the era of rounded, and, in the majority of the spe- 

 cies, of unornamented shells with simple septa ; the second is 

 the era of ornamentation, and the septa are steadily complicat- 

 ing ; in the third the complication of the septa, the ornamen- 

 tation, and the number of species, about twice that of any 

 other epoch, all combine to make it the zenith of development 

 in the order ; the fourth is distinguishable from all the preced- 

 ing as the era of retrogression in the form, and partially in the 

 septa. 



** The four periods of the individual are similarly arranged, 

 and have comparable characteristics. As has been previously 

 stated, the first is rounded and smooth, with simple septa ; the 

 second tuberculated, and the septa more complicated ; the third 

 was the only one in which the septa, form, and ornamentation 

 simultaneously attained the climax of individual complication ; 



deciduous or temporary dentition is not a remnant of the primitive dentition, but is 

 a later product of mammalian evolution. If it be a case of parallelism, it is inexact, 

 because the genus Protohippus was discovered by the writer to have three toes, 

 while Equus has but one. 



* " Memoirs Boston Soc. Nat. Hist.," 1866, p. 193. 



f He adds here : " Although in regard to geological sequence and structural po- 

 sition one of the extremes must be of higher geological rank." The " highest " ex- 

 treme will be of higher geological rank, according to the complexity of structure and 

 length of developmental scale, whether it come at the middle or end of the history 

 of the class in time. If, as has been the case, so far as known, a decline has termi- 

 nated the history of a class, its later forms are zoologically loicer than its older 

 ones. Hence the adjective high is only appropriate to types of the latter kind^ 

 when used as synonymous with extreme. 



