142 



SCIENCE. 



ADDRESS BY ALEXANDER AGASSIZ, 



Paleontological and Embryological Develop- 

 ment. 

 Since the publication of the " Poissons Fossiles " by 

 Agassiz and of the " Embryo! ogie des Salmonidees " by 

 Vogt, the similarity, traced by the former between certain 

 stages in the growth of3'oung fishes and the fossil represen- 

 tatives of extinct members of the group, has also been ob- 

 served in nearly every class of the animal kingdom, and 

 the fact has become a most convenient axiom in the study 

 of paleontological and embryological development. This 

 parallelism, which has been on the one side a strong argu- 

 ment in favor of design in the plan of creation, is now, with 

 slight emendations, doing duty on the other as a newly dis- 

 covered article of faith in the new biology. 



But while in a general way we accept the truth of the 

 proposition that there is a remarkable parallelism between 

 the embryonic development of a group and its paleontolog- 

 ical history, yet no one has attempted to demonstrate this, 

 or rather to show how far the parallelism extends. We 

 have up to the present time been satisfied with tracing the 

 general coincidence, or with striking individual cases. 



The resemblance between the pupa stage of some Insects 

 and of adult Crustacea, the earlier existence of the latter, 

 and the subsequent appearance of the former in paleonto- 

 logical history, furnished one of the first and most natural 

 illustrations of this parallelism ; while theoretically the 

 necessary development of the higher tracheate insects from 

 their early branchiate aquatic ancestors seemed to form 

 an additional link in the chain, and point to the Worms, 

 the representatives of the larval condition of Insects, as a 

 still earlier embryonic stage of the Articulates. 



Indeed, there is not a single group of the animal kingdom 

 in which embryology has not played a most important part 

 in demonstrating affinities little suspected before. The 

 development of our frogs, our salamanders, has given us 

 the key to much that was unexplained in the history of 

 Reptiles and Batrachians. The little that has been done in 

 the embryology of Birds has revolutionized our ideas of a 

 class which at the beginning of the century seemed to be 

 the most naturally circumscribed of all. Embryology and 

 paleontology combined have led to the recognition of a 

 natural classification uniting Birds and Reptiles on the one 

 side and Batrachians and Fishes on the other. It is to em- 

 bryology that we owe the explanation of the affinities of the 

 old Fishes in which Agassiz first recognized the similarity 

 to the embryo of Fishes now living, and by its aid we may 

 hope to understand the relationship of the oldest represen- 

 tatives of the class. It has given us the only explanation 

 of the early appearance of the Cartilaginous Fishes, and of 

 the probable formation of the earliest vertebrate limb from 

 the lateral embryonic fold, still to be traced in the young of 

 the Osseous Fishes of to-day. 



Embryology has helped us to ..nderstand the changes 

 aquatic animals must gradually undergo in order to become 

 capable of living upon dry land. It has given us pictures 

 of swimming-bladders existing as rudimentary lungs in 

 Fishes with abranchial system ; in Batrachians it has shown 

 us the persistence of a branchial system side by side with 

 a veritable lung. We find among the earliest terrestrial 

 Vertebrates, types having manifest affinities with the Fishes 

 on one side and Batrachians on the other, and we call these 

 types Reptiles ; but we should nevertheless do so with a 

 reservation, looking to embryology for the true meaning of 

 these half-fledged Reptiles, which lived at the period of 

 transition between an aquatic and terrestrial life, and must 

 therefore always retain an unusual importance in the study 

 of the development of animal life. 



When we come to the embryology of the marine Inver- 

 tebrates, the history of the development of the barnacles is 

 too familiar to be dwelt upon, and I need only allude to tin; 

 well-known transformations of the Echinoderms, of the 

 Acalephs, Polyps, in fact of every single class of Inverte- 

 brates, and perhaps in none more than in the Brachiopods, to I 

 -how how far-reaching has been the influence of embryology I 



in guiding us to a correct reading of the relations between 

 the fossils of successive formations. There is scarcely an 

 embryological monograph now published dealing with any 

 of the later stages of growth which does not speak of their 

 resemblance to some type of the group long ago extinct. 

 It has therefore been most natural to combine with the 

 attempts constantly made to establish the genetic sequence 

 between the genera of successive formations, an effort to 

 establish also a correspondence between their paleontolog- 

 ical sequence and that of the embryonic stages of develop- 

 ment of the same, thus extending the mere similarity first 

 observed between certain stages to a far broader generali- 

 zation. 



It would carry me too far to sketch out, except in a most 

 general way, even ior a single class, the agreement known 

 to exist in certain groups between their embryonic devel- 

 opment and their paleontological history. It is hinted at 

 in the succession of animal life of any period we may take 

 up, and perhaps cannot be better expressed than by com- 

 paring the fauna of any period as a whole with that of fol- 

 lowing epochs — a zoological system of the Jura, for in- 

 stance, compared with one made up for the Cretaceous ; 

 next, one for the Tertiary, compared with the fauna of the 

 present day. In no case could we find any class of the 

 animal kingdom bearing the same definitions or character- 

 ized in the same manner. But apply to this comparison 

 the data obtained from the embryological development of 

 our present fauna, and what a flood of light is thrown upon 

 the meaning of the succession of these apparently discon- 

 nected animal kingdoms, belonging to different geological 

 periods, especially in connection with the study of the few 

 ancient types which have survived to the present day from 

 the earliest times in the history of our earth ! 



Although there is hardly a class of the animal kingdom 

 in which some most interesting parallelism could not be 

 drawn, and while the material for an examination of this 

 parallelism is partially available for the Fishes, Mollusks, 

 Crustacea, Corals, and Crinoids, yet for the illustration and 

 critical examination of this parallelism I have been led to 

 choose to-day a very limited group, that of Sea-urchins, 

 both on account of the nature of the material and of m> 

 own familiarity with their development and with the living 

 and extinct species of Echini. The number of living 

 species is not very great — less than three hundred — and the 

 number of fossil species thus far known is not, according 

 to Zittel, more than about two thousand. It is therefore 

 possible for a specialist to know of his own knowledge the 

 greater part of the species of the group. It has been my 

 good fortune to examine all but a few of the species now 

 known to exist, and the collections to which I have had 

 access contain representatives of the majority of the fossil 

 species. Sea-urchins are found in the oldest fossiliferous 

 rocks; they have continued to exist without interruption in 

 all the strata up to the present time. While it is true that 

 our knowledge of the Sea-urchins occurring before the 

 Jurassic period is not very satisfactory, it is yet complete 

 enough for the purposes of the present essay, as it will 

 enable me, starting from the Jurassic period, to call your 

 attention to the paleontological history of the group, and to 

 compare the succession of its members with the embryo- 

 logical development of the types now living in our seas. 

 Ample material for making this comparison is fortunately 

 at hand ; it is material of a peculiar kind, not easily ob- 

 tained, and which thus far has not greatly attracted the at- 

 tention of zoologists. 



Interesting and important as are the earliest stages of 

 embryonic development in the different classes of the ani- 

 mal kingdom, as bearing upon the history of the first ap- 

 pearance of any organ and its subsequent modifications, 

 they throw but little light on the subject before us. What 

 we need for our comparisons are the various stages of 

 growth through which the young Sea-urchins of different 

 families pass from the time they have practically become 

 Sea-urchins until they have attained the stage which we 

 now dignify with the name of species. Few embryologists 

 have carried their investigations into the more extended 

 field of the changes the embryo undergoes when it begins 

 to be recognized as belonging to a special class, and when 

 the knowledge of the specialist is absolutely needed to 

 trace the bearing of the changes undergone, and to under- 

 stand their full meaning. Fortunately the growth of the 



