EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 357 



that no other classification or order of succession could take 

 place by natural descent. 



Evolution of the Suture results in the improvement of the 

 Structure of the Shell. When we look at the complex foliated 

 septum of the Ammonite in relation to its use, we are struck 

 with the economical use of materials for greatest strength 

 with least weight. The principle of using thin plates of 

 corrugated material in place of solid supports in engineering 

 and building is well understood by man, and from this point 

 of view it appears evident that the result of the evolution of 

 the cephalopod septum has been the improvement of the 

 device concerned. 



In conclusion, the analysis of the structure of the Cepha- 

 lopoda, based upon a comparison of the different modifica- 

 tions of their structure and upon the historical study of the 

 fossil remains of this class of animals, shows very clearly that 

 there is an intimate co-ordination between (a) the morpho- 

 logical differentiation of the characters, and (U) the historical 

 sequence of initiation and of dominance in numbers of the 

 individuals exhibiting them. Thus we notice, upon exami- 

 nation of the characters of the two great divisions Tetrabran- 

 chiata and Dibranchiata, that the group which appeared 

 later, and after the first had flourished and the great majority 

 of its families and genera had become extinct, was the one in 

 which is found the greater amount of differentiation of each 

 of the characters by which the two groups are distinguished. 



It is also to be observed that, among the characters, in- 

 cluding all that is known of the group, by which the grand 

 divisions of the Tetrabranchiata are discriminated those which 

 were less differentiated morphologically were first to appear. 

 In the case of the modification of the sutures, about which the 

 facts have been minutely studied, the types follow each other 

 in regular successive order from the less differentiated to the 

 more highly differentiated ; and the same order is observed in 

 the numerical dominance of the several types. We notice 

 also that this order of increasing differentiation, which may 

 be traced in the case of the suture of the Ammonoidea, is 

 the natural order of evolution when viewed from the points 

 of view of (a) mechanical differentiation, that is, the greatest 



