138 TEE POPULAR SCIENCE MONTHLY 



usually lost after the chambered shell grows out from it, but in the 

 aramonoids the protoeonch is hard and calcareous and is always found 

 at the narrow end of the series of chambers composing the shell. 



The undisputed facts are that in Silurian times straight conical 

 shells with smooth outer surface were common, and coiled shells were 

 rare. In the Carboniferous the coiled shells gain in number and no- 

 dules, ribs or keels begin to develop upon their outer surfaces. In the 

 Jurassic we find only close-coiled, or uncoiling shells, and those that 

 uncoil tend to become straight with smooth surfaces, as were their 

 ancestors long ago in Silurian times. The ammonoids arose as coiled 

 forms from the nautiloids in Cambrian times, but in the Jurassic and 

 Cretaceous they uncoil and thus resemble their straight-shelled nauti- 

 loid ancestors, the oldest of their race. Uhlig, Neumayr, Zittel, Hyatt 

 and all other students of the group agree upon these points, and indeed 

 Hyatt's observations of fact have won high respect for both their ac- 

 curacy and their number. It is only in matters of inference that he is 

 at variance with many zoologists. 



Hyatt found that in the very young ammonoids the shell is at first 

 straight and smooth, then as growth proceeds it coils upon itself, and 

 may acquire a keeled, ribbed or nodular surface. Finally, if the species 

 be a Jurassic or Cretaceous form it is apt to uncoil in later life, and 

 the uncoiled part of the shell tends to become smooth and relatively 

 straight like its own young stage. D'Orbigny observed these facts 

 even before Hyatt, and they are well authenticated by numerous stu- 

 dents of the group. Hyatt, however, pointed out the interesting fact 

 that there is a parallelism between the growth-stages of the individual, 

 and the genetic succession of species through which the race has devel- 

 oped. For example, the young shell is smooth and straight as were 

 the adult shells of its primitive nautiloid ancestors of Cambrian times. 

 The adult shell is coiled and ornamented as were those of the ammonites 

 of the Devonian when the race was dominant. The old shell is again 

 straight and smooth as were its Cambrian ancestors and the vanishing 

 remnants of the race that died out in upper Cretaceous times. Thus 

 the growth-phases of the individual — embryonic, larval, adolescent, 

 adult and old age — are correlated with the changes which occur in the 

 geological history of the group to which it belongs. 



It may be a mere coincidence, but certainly in ammonites there is 

 a surprisingly close correspondence between the growth-stages of the 

 individual and the phylogeny of its race. Hyatt believes that this 

 fact is not due to accident, but that the life of the individual and the 

 life of the race are related and obey one and the same law. 



Zoologists can not understand why this must necessarily be so, but 

 there are many laws in nature which man has discovered, and the logical 

 necessity for which we have not yet understood, and its incomprehensi- 

 bility has naught to do with the truth or falsity of Hyatt's theory. 



