WEAVER: INVERTEBRATE PALEONTOLOGY AND HISTORICAL GEOLOGY 697 



lustrations of genera and species. The morphology of the hard parts of different 

 groups of invertebrate organisms was now well known; also the facts connected 

 with the occurrence of fossils in successive layers of sedimentary rocks. Infor- 

 mation concerning their geographic distribution and their relation to changing 

 lithologic facies caused many investigators to accept partially the new concept 

 of mutability of species. Those who now did so no longer considered a species 

 as confined to a single formation or interval of geologic time but held that cer- 

 tain species of a fauna might survive and continue to exist in the next succeed- 

 ing set of strata. These concepts profoundly modified the ideas previously held 

 by scientific men in the fields of paleontology, geology, and natural history. 



Speculative concepts of the principles of evolution had been expressed before 

 but were not generally supported by scientific facts. Paleontological research 

 was now endeavoring to inquire into the nature of evolutionary processes based 

 on a growing knowledge of the succession of faunas through geologic time. 

 Charles Darwin began his investigation of the origin of species during this inter- 

 val of change in scientific thought and in 1859 his monumental work on The 

 Origin of Species by Means of Natural Selection appeared. The ideas expressed 

 in this work appealed strongly to H. G. Bronn, who had already considered that 

 transitional genera and species in a fauna of a given time interval might extend 

 across the gap into the next succeeding interval. He translated Darwin's work 

 into German and it immediately influenced the thought of those scientists on 

 the Continent who were concerned with paleontological investigations. Darwin, 

 in attempting to account for the method of action in the theory of evolution, 

 placed emphasis on those factors involved in environment, including variations 

 in climate, temperature, accessibility of food supplies, and means of protection 

 from other organisms living in the same environment. He considered also how 

 these factors singly or jointly might react favorably or unfavorably on each in- 

 dividual, depending on how each particular feature of the anatomy might re- 

 spond. Morphological characters of a varietal nature were considered to influ- 

 fiuence the individual's response to the varying stimuli of the environment; thus 

 those individuals with the most favorable variations would survive. This con- 

 cept was designated as the law of natural selection. The principles of sexual 

 selection were also included. 



Before leaving for North America in 1847 Louis Agassiz had published the 

 results of ten years of research on fossil fishes. Emphasis was laid on genealogy, 

 including progressive morphological changes of certain parts of the skeleton as 

 observed in different species and genera in passing from older to younger time 

 intervals. Agassiz showed that information from the phylogenetic history could 

 be of great value to geology for the systematic classification of sedimentary de- 

 posits. He also pointed out the correspondence of certain changes in the embry- 

 onic development of an individual to those exhibited in the phylogeny during 

 geologic time. During the second half of the nineteenth century, as a lecturer 

 in Harvard University, he inspired many of his students with the spirit of re- 

 search and the inclination to search further for more detailed factors involved 

 in the process of evolution as shown in fossil organisms. 



The detailed morphological study necessary to yield sufficient data for ex- 

 panding these principles of evolution of organic life during past geologic epochs 

 required that the investigator devote special attention to the fossils of a single 



