246 FOOT-NOTES TO EVOLUTION. 
stock of unspecialized Prolecanitide endured as long as 
the race. The number of phylogerontic forms increases 
in the Mesozoic, showing a constantly increasing tend- 
ency to become abnormal, until before the end of the 
Cretaceous the entire race of ammonoids becomes phy- 
logerontic, and dies out from sheer lack of plasticity to 
modify itself further with changing conditions. 
Such a general view or family tree of the ammonoids 
may be seen in any of the text-books of paleontology, 
especially those of Steinmann,* and of K. von Zittel,t 
where we get the best attempts to represent our present 
knowledge and ideas of the genetic relationships of am- 
monites. These genealogies are, however, purely ten- 
tative, based not on ontogeny but on comparison of 
series of adults. This would undoubtedly be the safest 
way if we had a perfect series of genera and species, but 
such a thing is unknown, and can never be obtained, on 
account of the incompleteness of the geologic record, 
and the mixing of faunas by migration in the past. 
The researches of Hyatt, Branco, and Karpinsky 
have given us a surer way; from their work we have 
learned that the Ammonoidea preserve in each individual 
a complete record of their larval and early adolescent 
history, the embryonic protoconch and the young cham- 
bers being enveloped and protected by later stages of 
the shell. Also the record is a perfect one, for no 
resorption of stages of the shell has ever been observed 
in the chambered cephalopods. And so by breaking off 
the outer chambers the naturalist can in effect cause the 
shell to repeat its life history in inverse order, for each 
stage of growth represents some extinct ancestral genus. 
These genera appeared in the exact order of their 
minute imitations in the larval history of their descend- 
* Elemente der Palzontologie, ‘1890. 
t Grundziige der Palzontologie, 1895. 
