54 MICHIGAN ACADEMY OF SCIENCE. 



fundamental facts upon which rest the whole structure of the modern 

 science. For, if the accepted theories of the creation of the world and 

 the evolution of the life upon it be true, there can be no doubt that the 

 first land which appeared above the surface of the primordial ocean was 

 peopled from the waters that gave it birth, and that to the marine forms 

 of life must be traced the origin of all existing forms of animated nature. 

 According to Bronn, "the principal change affecting the external condi- 

 tions of the existence of animal life is to be found in the progressive 

 development of the surface of the earth, in the subdivision of the univer- 

 sal primordial ocean into great inland seas and "in the elevation of the 

 plateaus and ranges of mountains. Simultaneously a correlative change 

 manifests itself in the organic world. To the original fauna exclusively 

 pelagic and natatory is added first a deep sea fauna, then a littoral one 

 in the shallow waters, and finally one inhabited exclusively the land." 

 But while this is the accepted theory and notwithstanding the enormous 

 amount of facts, which have been already accumulated to substantiate 

 it, there are yet wanting in almost every department, owing partially to 

 the imperfection of the geological record, and partially to the magnitude 

 of the work involved, many links necessary to complete the genealogy of 

 existing forms of life, and in no class of the animal kingdom is the break 

 as complete as in the phylogeny of the land and fresh water mollusca. 

 The earliest forms of terrestrial mollusks yet known are from the Carbonif- 

 eronus deposits of North America. The fresh water univalves first 

 appear in the rocks of the upper Jurassic, as do also the Cyrenidae. 

 The Unionidpe do not appear until the lower Cretaceous, although cer- 

 tain forms as yet imperfectly known, but which may be connected with 

 them have been described from the Carboniferous period. But in all 

 cases the families and genera are fully differentiated and substantially 

 identical with those now in existence. And although this fact indi- 

 cates that these forms must even then have had a long existence in 

 order to have acquired such a high degree of differentiation, there is yet 

 a total lack of the earlier ancestral and more primitive types connecting 

 them with the marine forms. By the study of the embryology, anatomy 

 and morphology of existing forms, we can more or less clearly arrive at 

 theoretical conclusions as to their relations with the marine mollusca 

 and the probable line of descent, and from the habits and mode of life 

 of recent mollusks can postulate theories more or less satisfactory as to 

 the manner in which these great changes were brought about. But more 

 than that is now and will be impossible until the earth shall yield the 

 secret to her inquiring children. But putting speculation aside and 

 relying wholly upon the palaeontological evidences already in our pos- 

 session, we find that from the time these primitive mollusks first ap- 

 peared, they have existed in constantly increasing numbers and with a 

 greater degree of specialization in each succeeding epoch. And by a 

 comparative study of the fossil and recent forms both in their phy- 

 logenetic relations and their distribution in time and in connection with 

 the theories of the geologists in regard to the successive changes in the 

 earth's surface, we can frequently trace back the history of many of our 

 recent species to remote times and satisfactorily account for their present 

 oftimes seeming erratic distribution. For when the chronological 

 order of the appearance of the different families and genera coincides 



