i8z PRIMARY FACTORS OF ORGANIC EVOLUTION. 



2. PARALLELISM IN THE CEPHALOPODA. 



Among Mollusca it is well known that the Cepha- 

 lopoda form a number of series of remarkable regular- 

 ity, the advance being, in the first place, in the com- 

 plication of the folds of the external margins of the 

 septa, and, in the second place, in the degree of invo- 

 lution 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 sub- 

 ject,i points out that the less complex forms are in 

 many cases identical with the undeveloped conditions 

 of the more complex. He says : "There is a direct 

 connection between the position of a shell, in the com- 

 pleted cycle of the life of this order, and its own de- 

 velopment. Those shells occupying the extremes of 

 the cycle" (in time), "the polar forms, being more 

 embryonic than the intermediate forms. The first 

 epoch of the order is especially the era of rounded, 

 and, in the majority of the species, of unornamented 

 shells with simple septa ; the second is the era of or- 

 namentation, and the septa are steadily complicating; 

 in the third the complication of the septa, the orna- 

 mentation, 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 dis- 

 tinguishable from all the preceding as the era of re- 

 trogression in the form, and partially in the septa. 



"The four periods of the individual are similarly 

 arranged, and have comparable characteristics. As 



^Memoirs of the Boston Society for Natural History, 1866, p. 193. Hyatt 

 was followed by WQrtenberger in Ausland, 1873, who entirely confirmed his 

 conclusions. 



