254 INVERTEBRATE ZOOLOGY 



Any attempt at classification in some measure endeavors to 

 convey a concept of relationships. Groupings or separations are 

 based upon an evaluation of the presence or the lack of common 

 characteristics. In most instances, the possession of fundamen- 

 tal characters of like nature is considered as indicative of common 

 origin. As pointed out in Chapter I, the student is apt to con- 

 sider phylogenetic relationships from an erroneous point of view, 

 assuming that such kinship is traced between the respresentatives 

 of present-day groups when as a matter of fact all of our existing 

 organisms have undergone some degree of differentiation since 

 their origin. It is only through common ancestry that these 

 lines of kinship are traced. 



Sources of Evidences of Relationships.^ — Obviously, then, 

 relationships are not observable directly but frequently may be 

 deduced from evidences gathered from one or more of the fields 

 of paleontology, embryology, and comparative anatomy. Even 

 these available sources in many instances give incomplete and 

 inconclusive evidence of an organism's past history. Present-day 

 organisms are but points in the lines of evolutionary progress. 

 The source and the termination of any such line are but rarely 

 discovered. Through the study of paleontology and embryology 

 we gain a glimpse here and there of portions of the path which 

 various organisms have followed in reaching their present posi- 

 tion in the lines of evolutionary progression. 



Paleontology. — In determining a broad outline of the course 

 of phylogeny among the invertebrates, the science of paleontology 

 offers but fragmentary and woefully incomplete data concerning 

 the ancestral forms. The Cambrian period has disclosed repre- 

 sentatives of most of the important invertebrate phyla, yet, 

 as Schuchert has said, more fundamental evolution had taken 

 place up to this time than subsequently. Studies of the rocks 

 of the preceding periods have revealed rare and imperfect and 

 almost indecipherable evidences of a marine fauna including 

 Protozoa and marine worms. Careful studies by Walcott and 

 others have furnished evidences that practically all of the impor- 

 tant invertebrate phyla were represented in the fauna of pre- 

 Cambrian times. However, the pre-Cambrian rocks, have under- 

 gone metamorphosis under the action of heat and pressure 

 until their contained fossil remains have become concealed or 

 even destroyed. 



