Page:Popular Science Monthly Volume 77.djvu/156

150 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 Ginkgoales, 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 understandable 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.



HE 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 development, 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 individual, and thus no longer give us a true image of bygone structure. These cœnogenetic 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