CLASSIFICATION BY DESCENT. 135 



order, or that one could not take place without involving the necessity for the other 

 which I have set down as dependent upon it. We have no reason to be surprised 

 if we found great involution unaccompanied by any addition to the suture-line ; but 

 we can perfectly well understand that such a form of shell would not be so well 

 able to resist pressure as an evolute form with the same suture-line, or as an 

 involute form with more complex sutures. In the same way the ventral area may 

 have become very narrow and may yet retain its furrows ; but it is obvious that they 

 must decrease in size, and that they will tend to become extinct. 1 



The advance towards involution may not be at all regular in different species. 

 We may find compressed, broadly-whorled forms, with a large spiral angle exposing 

 a large umbilicus ; or we may observe the same shaped whorls with a lesser spiral 

 angle and a small umbilicus, and consequently with greater inclusion. At the 

 same time we do not find the auxiliary lobes developed before the inclusion had 

 become great ; we may find, and do find, species with great inclusion which have 

 not yet developed the auxiliary lobes to support the extra space — that is a later 

 process. Again, we may find a later, but yet more evolute, form of the same 

 genus possessing the same number of auxiliary lobes as the involute forms. Here, 

 through partial reversion, we have a wider umbilicus — and therefore narrower 

 whorls ; but the animal has been unable to get rid of the extra, though it may 

 be useless, auxiliary lobes which it has inherited, and consequently they must 

 be cramped, or the other lobes and saddles must be decreased in size {Hyperlio- 

 ceras Walheri). 



The lessons which the Hildoceratldce and their allies have taught me 

 are what I have tried to set down above. In doing so I have endeavoured 

 to keep in view only the evidence given by the Ammonites themselves, 

 and not put forward anything unless it was a surmise which could fairly be 

 deduced from the sequence of changes observable in them. 



It follows from these remarks upon the development of the Hildoceratidce that 

 the further we trace the three branches back the more nearly we should find them 

 alike, and therefore that there would be a great desire to unite, perhaps in one 



permitted, each character that it had inherited. Practically speaking the changes from one genera- 

 tion to another would be so small as to be imperceptible ; but a series culled from many life-zones 

 shows the various stages of change. In some genetic series the changes appear to have succeeded 

 each other with great rapidity ; in others the individuals may pass through several life-zones without 

 any great alteration ; the period of activity among the members of one series may be the period of 

 quiescence among those of another ; periods of activity and of quiescence will probably be found to 

 alternate in the same series. Alterations in the conditions of life would accelerate or retard these 

 changes, and thus account for such periods. 



1 The adults of Hildoceras exhibit less marked furrows than the younger specimens. By 

 Rule 3 the furrows would become extinct in time. The adults of all these species have whorls more 

 compressed in proportion than when younger, and by Rule 1 c this would help to cause what is stated 

 in the preceding sentence, while by Rule 3 it would always be becoming more intensified. 



