DEVELOI'MKNT OF AMMONITES. 213 



first. Thus searching through the strata from below 

 upwards, we see modification after modification begin- 

 ning at the outer part of the Ammonites, and advancing 

 towards the centre of the discs. The innermost convo- 

 lutions often resist these innovations with great persis- 

 tency, so that we usually fmd upon tlicir surface several 

 of these states of development in close juxta-position, 

 as the shell of the individual Ammonite begins with 

 the old morphological type, and then adopts the modifi- 

 cations in the same order in which they follow in vast 

 periods in the geological development of the groups con- 

 cerned." 



"The Ammonites," he says moreover, "thus obtain 

 at an advanced and maturer age — only when they have 

 gone through the development inherited from their 

 parents, and as much as possible in the same manner as 

 tneir parents — the power of modifying themselves in a 

 new direction, tliat is to say, of adapting themselves to 

 new conditions ; yet these modifications may then be 

 transmitted to the offspring, so as to appear in each 

 subsequent generation a trifle earlier, until this phase of 

 development in its turn characterizes the greater portion 

 of the period of growth. But this last and longest 

 phase of development scarcely ever suffers itself to be 

 supplanted by new ones, formed in like manner; heredity 

 operates so powerfully, that a period of development 

 thus once predominant, is repeated in the infancy of the 

 Ammonites, even though but slightly indicated. Hence 

 in an individual Ammonite from a recent stratum, the 

 periods of development compressed and forced back 

 upon the innermost convolutions, must appear in the 

 same succession in which they wrested the dominion 



