No. 556] 



ALPHEUS HYATT 



197 



vidual gives in abbreviated form a recapitulation of the 

 phylogeny of the group. This is the law of morpho- 

 genesis of Hyatt 2 by which he endeavored to demonstrate 

 that a natural classification may be made by a system of 

 analysis in which the individual is the unit of compari- 

 son, because its life in all its phases, morphological and 

 physiological, healthy or pathological, embryo, larva, 

 adolescent, adult and old (ontogeny), correlates with 

 the morphological and physiological history of the group 

 to which it belongs (phylogeny). 



To the student of invertebrate fossils these animals 

 present certain advantages, not only on account of their 

 relative abundance, but also because, in many forms at 

 least, from the study of a single specimen, one can gather 

 in a more or less complete degree the stages through 

 which it has passed in development. As Hyatt 3 says : 



How unreasonable it would seem to a student of fossil Mammalia, if 

 he were requested to do what it would be appropriate to require from 

 a student of fossil Cephalopoda, viz., to describe from the investigation 

 of a single perfect fossil skeleton of an adult, not only the character- 

 istics of the skeleton at the stage of growth at which the animal died, 

 but the developmental stages of this same skeleton, and in case it were 

 the remains of an old, outgrown animal, also, the retrograde meta- 

 morphoses through which it had passed during it- last stages of decline. 

 It might require a lifetime to make out the stages of a single species 

 of mammal satisfactorily from the isolated specimens which would be 

 found and the attempt would be hopeless for all the youngest stages of 

 growth, while the bones'were still cartilaginous. This kind of evidence, 

 however, is readily obtainable among fossil Cephalopods . . . and it 

 can be obtained in good collections everywhere. 



While this is especially true of the tetrabranchiate 

 cephalopods, it is also true in a more or less complete 

 degree of some other groups of molluscs, as well as many 

 brachiopods, echinoderms and corals. 



As examples of types showing stages in development, 

 the following may be cited. The living Nautili** lias a 

 close-coiled shell, but in its development passes through 



Knowledge, Washington, 1889. 



S A. Hyatt. "Phylogenv of an Acquired Characteristic," Proc. Amer. Phil. 

 Soe., Vol. 32, 1894. 



