150 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 Professor RICHARD SWANN LULL 



YALE UNIVERSITY 



THE 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 ccenogenetic 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 



