268 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



ostrean form is the base of a series, from the summit 

 of which we get a clearer view." 



c. Mechanical Origin of the Impressed Zone in Cepha- 

 lopoda. 



Prof. Alpheus Hyatt has shown that the groove on 

 the dorsum or inside of each coil of the Cephalopoda 

 is due to the pressure exercised by contact with the 

 ventral side of the coil within it. He has shown that 

 this groove persists in cases where the shell in the 

 course of evolution has become more or less unwound, 

 and he regards this as an example of the inheritance 

 of a mechanically acquired character. This subject is 

 presented in greater detail in the part of this book de- 

 voted to heredity. 



3. KINETOGENESIS IN VERMES AND ARTHROPODA. 



It is believed with good reason that the Arthropoda 

 have descended from some of the forms included in 

 the branch Vermes, and perhaps Peripatus furnishes 

 the nearest living approach to that type. The ances- 

 tor, whatever it may have been, developed limbs from 

 processes of the body-wall, and used them to aid in 

 progression. Peripatus has soft flexible limbs, and a 

 non-chitinous integument generally. With the begin- 

 ning of induration of the integument, segmentation 

 would immediately appear, for the movements of the 

 body and limbs would interrupt the deposit at such 

 points as would experience the greatest flexure. The 

 muscular system would initiate the process, since flexure 

 depends on its contractions, and its presence in ani- 

 mals prior to the induration of the integuments in the 

 order of phylogeny, furnishes the condition required. 

 It is a matter of detail how the diverse segmentations 



