224 INFERIOR OOLITE AMMONITES. 



pincli of hard times, it was not these involute forms which continued to perpetuate 

 the Ammonite-race. They had to yield place to the evolute species of the other 

 genus. In turn the new immigrants passed through the same changes, on account 

 of similar causes, and with very similar results. 



Finally, I have one more point to mention — a point which concerns Ammonites 

 generally, but the Hildoceratidge more particularly, at present. At p. 159, foot- 

 note, I have alluded to the manner in which all traces of an intermediate stage of 

 development may be omitted from the inner whorls. Assuming a tendency to 

 dispense with a stage dissimilar to its predecessor or successor so as to avoid a 

 useless change, yet it seems probable that this tendency might not be very powerful 

 until the dissimilar stage had, owmg to the encroachments of its successor, become 

 very unimportant. It seems to me that, allowing all the characters of the parent 

 to have been inherited at an age ever so slightly earlier in each successive genera- 

 tion (p. 134, foot-note), it is necessary also to suppose that the later-acquired 

 character always made the most progress in this matter; and, therefore, each 

 preceding character made a progress gradually slower in proportion to the remote- 

 ness of the time when it had been acquired. 



Allowing the fact of "earlier inheritance" in the descendant, then each 

 generation must add something of its own. If the descendants continued the 

 development in the same course as their parents, then the additions made would 

 be practically imperceptible ; but if gradual modifications were being introduced, 

 no matter how slowly, then in a certain number of generations there would be an 

 appreciable change — we should have a new " stage." The result I wish to arrive 

 at is that the characters, whether perceptible or otherwise, which originate with 

 the parents make greater progress in the matter of " earlier inheritance " than 

 those derived from the grandparents ; and these again make greater progress than 

 those derived from the great-grandparents, and so on. Applying this idea to what 

 are called the " different stages," which are really the perceptible additions, then 

 the last stage makes greater progress than the penultimate stage, and so on ; or, 

 stated as a general law, the " earlier inheritance " of a given character is always 

 diminishhig . Of necessity the omission of the characters of any stage, that is, its 

 obliteration by its encroaching successor, must depend upon the number of 

 generations through which the stage endured without change — the greater the 

 number the longer the time required ; but, as I have said, we may also suppose 

 an acceleration of the process towards the last if what had come to be a useless 

 change could thereby be avoided. 



The theory above enunciated would account for the persistence of the 

 characters of innermost whorls, because, being derived from ancestors so very 

 remote, their progress in the matter of " earlier inheritance " would be practically- 

 speaking, though not actually, nil. 



