16 
of existing forms of life, supplemented by 
that of extinct forms. Almost from the 
start, however, it was evident that the data 
derived from those sources were sufficient 
without the additional evidence afforded 
by the facts of embryological development. 
Despite the high degree of validity pos- 
sessed by the paleontological evidence, the 
record is, and is likely always to remain, 
too meager to guide us to the broader 
results we seek. Without the aid of em- 
bryology, comparative anatomy, with all 
its wealth of data, gives us hardly a hint of 
some of the most fundamental relations of 
living things. The high value of the em- 
bryological evidence was therefore early 
recognized ; and with the progress of re- 
search it played a more and more important 
role in the examination of genealogical 
problems. 
It seems a singular irony of fate that 
Agassiz, an anti-evolutionist, should have 
singled out as the most important result of 
his life-work a discovery in embryology, 
which, in connection with the generaliza- 
tions of von Baer and Darwin, was destined 
to form one of the watchwords of a coming 
generation of evolutionists. ‘I have de- 
voted my life to the study of Nature, and 
yet a single sentence may express all that I 
have done. I have shown that there is a 
correspondence between the succession of 
forms in geological times and the different 
stages of their growth in the egg—this is 
all.” In another place he urges young stu- 
‘dents to turn to the study of embryology ; 
for here, he says, lies ‘an inexhaustible 
mine of valuable information—where we 
shall find the true facts by which to deter- 
mine the various kinds and different de- 
grees of affinity which animals bear not 
‘only to one another, but also to those that 
have preceded them in past geological 
times.’ How little he foresaw the use 
which embryologists were soon to make of 
this principle or the lengths to which they 
SCIENCE. 
[N. 8. Vou. XIII. No. 314. 
would go in its application. Jt was in that 
very year that Feitz Muller published the 
famous little book entitled ‘ Facts for Dar- 
win,’ which contained the first clear outline 
of the recapitulation theory and marked the 
beginning of the embryological search for 
genealogies, continued with so much ardor 
by Haeckel, Semper, Claus, Dohrn, Balfour 
and a hundred others. Many of us have 
eagerly followed the phases of that long 
quest or have sought to make our own 
modest contributions to it. We know how 
many puzzling problems of comparative 
morphology it has brought to a solution, 
how great an impulse was given to the 
investigation of natural affinities by the 
formulation of the recapitulation theory by 
Muller, Haeckel and their followers. I 
would be the last to question the immense 
interest and value of the results that have 
thus been achieved in the field of genealog- 
ical inquiry. And yet I believe that when 
these results, together with those derived 
from all other sources, are broadly viewed, 
we are constrained to the admission that 
comparative morphology as a whole has 
thus far solved only minor problems of 
descent, and that naturalists as a body are 
beginning to turn their attention in other 
directions. Let any one who doubts this 
compare the present attitude of naturalists 
towards some of the more general problems 
of descent with that of fifteen or twenty 
years ago. At that time the burning ques- 
tions of zoological morphology centered in 
far-reaching genealogical hypotheses such as 
the Gastrea theory, the Trochophore theory, 
the Nauplius theory, the origin of verte- 
brates, the origin of metamerism, or the ~ 
derivation of bilateral animals from medu- 
soid or polypoid forms. They still remain 
questions of very high interest, but they 
are no longer the leading questions of the 
day ; and we may as well admit the truth 
that interest in them is beginning to wane, 
temporarily perhaps, but unmistakably. 
