to show how much we owe to paleontology. There is not a single sub-kingdom but has been immensely enriched from this source.
Some of the fossil species possess morphological characters so closely allied, on the one hand to earlier, and on the other to later, forms as to indicate that they occupy a position in the line of descent, and phylogenetic series have been established frequently on this basis. As examples we have the well-known developmental series of the horse and the camel. Other illustrations may be found in the Paludinas of the Slavonian Pliocene and in the Planorbis types of Steinheim.
Still other fossil forms combine in the same species several morphological features which later become segregated and characterize different types. Such "synthetic types" serve to show the common origin of the forms in question if not their actual ancestors and have greatly enlarged our knowledge of the morphology of the several groups involved. These early forms are, for the most part, highly generalized, while their descendents are variously specialized. Take, for example, the mammalian Condylartha, small, generalized Ungulata with an astragalus shaped almost as in the Carnivora; or the reptilian Anomodontia with intermediate skeletal characters between the highest labyrinthodonts and the lowest mammals; or again, the early Paleozoic cystoids with generalized characters in their calyx plates which appear in more specialized forms in later crinoids and blastoids. An almost indefinite number of such illustrations might be cited.
Still other fossil forms present morphological characters so different from other fossil or living species that the genetic relationships may not be determined accurately. Some of these are possible of reference to already defined orders, while others present so many diverse morphological characters as to require the establishment of new divisions for their reception.
A survey of the known fossil and living forms shows that not only have old species constantly become extinct during the progress of geological time, but new species have been as frequently appearing. This is equally true of genera, families, orders or even classes. Some forms have appeared and disappeared, as the case may be, suddenly; others slowly. The great group of the Ammonites, for example, disappeared suddenly at the close of the Cretaceous after showing many degenerate characters, while the Trilobites gradually declined during late Paleozoic time before their final extinction. One of the most striking features in the developmental history of plants and animals is found in the great number of fossil types which have left no descendants.
Both the animal and plant kingdoms furnish a wealth of material with which to demonstrate the aid which paleontology has rendered to morphology.
The contributions of invertebrate paleontology are numerous and striking:
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