142 
SCIENCE 
ADDRESS BY ALEXANDER AGASSIZ, 
Paleontological and Embryological Develop- 
ment. 
Since the publication of the “ Poissons Fossiles ” by 
Agassiz and of the “ Embryologie des Salmoniddes ” by 
Vogt, the similarity, traced by the former between certain 
stages in the growth of young 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 earl)' 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 Batraclnans 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 understand 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 the 
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 
show 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 tor a single class, the agreement known 
to exist in certain groups between their embryonic devel- 
j 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 
I 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 
