25° THE POPULAR SCIENCE MONTHLY 
groups of plants which have made similar great contributions to 
morphology. The Cycadales or cycad-like plants, which to-day are an 
inconspicuous group, were one of the dominant Mesozoic types, and any 
understanding of the modern forms rests entirely upon a study of their 
immensely abundant Mesozoic ancestors. The other group, the Gink- 
goales, represented in the existing flora by a single species, the ginkgo, 
is found in the Mesozoic to have been represented by many genera and 
species of great diversity. 
The dominant plants of to-day, the conifers on the one hand, and 
the angiosperms on the other, have each afforded many extinct genera, 
the former with more fossil than recent species, and only understand- 
able in the light of their fossil ancestors. Vegetable morphology based 
only upon existing plants abundantly demonstrated its sterility before 
the relative recent study of fossil plants placed it upon an altogether 
new basis. 
RELATION OF EMBRYOLOGY AND VERTEBRATE 
PALEONTOLOGY 
By Prorgessor RICHARD SWANN LULL 
YALE UNIVERSITY 
5 Ge problem of recapitulation among vertebrates gives by no means 
. as accurate results as among invertebrate forms, for while a single 
adult shell, if perfectly preserved, will often display the entire life 
history or ontogeny of the individual, a bone, or even a complete 
skeleton, is rarely retrospective and if at all only in some minor detail. 
The vertebratist, therefore, in his study of ontogeny, for comparison 
with racial history must needs follow either the entire growth of one 
animal, a thing manifestly impossible when the embryonic stages are 
considered, or study a long series of individuals in various stages of de- 
velopment, the securing of which in the great majority of cases is largely 
the result of a number of happy accidents. When one comes to weigh 
the evidence offered by the actual embryos of fossil vertebrates he will 
find a very great dearth of material, for fossil embryos—that is, the 
stages in the life history before birth or hatching—are extremely rare. 
Recent embryology, on the other hand, is more productive of results 
and the earlier stages of certain organs often suggest those of equivalent 
development in animals of the past. In his interpretation of a given 
structure, however, one has to bear in mind whether it may not have 
been modified to suit some modern need in the life history of the indi- 
vidual, and thus no longer give us a true image of bygone structure. 
These coonogenetic organs are not historic, but as Wilder says, “ have 
to do with such immediate environmental problems as nutrition or 
protection.” Again, if the organ has approximately the same form 
