146 TEE POPULAR SCIENCE MONTHLY 



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 phylo- 

 genetic series have been established frequently on this basis. As ex- 

 amples we have the well-known developmental series of the horse and 

 the camel. Other illustrations may be found in the Pdludinas of the 

 Slavonian Pliocene and in the Planorbis types of Steinheim. 



Still other fossil forms combine in the same species several morpho- 

 logical 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 in- 

 volved. These early forms are, for the most part, highly generalized, 

 while their descendents are variously specialized. Take, for example, 

 the mammalian Condylartlia, small, generalized Ungulata with an 

 astragalus shaped almost as in the Carnivora; or the reptilian Anomo- 

 dontia 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 dif- 

 ferent from other fossil or living species that the genetic relationships 

 may not be determined accurately. Some of these are possible of refer- 

 ence to already defined orders, while others present so many diverse 

 morphological characters as to require the establishment of new divi- 

 sions 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, dis- 

 appeared 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 : 



