392 A CENTUEY OF SCIENCE 



zoologists have been recorded in the Biographical 

 Memoirs of the National Academy of Sciences. 



The developmental history of zoology in America falls 

 naturally into four fairly well marked periods, namely : 

 1, Period of descriptive natural history, previous to 

 1847, embracing the early studies on the classification 

 and habits of animals, characteristic of the zoological 

 work previous to the arrival of Louis Agassiz in Amer- 

 ica. 2, Period of morphology and embryology, 1847- 

 1870, during which the influence of Agassiz directed the 

 zoological studies toward problems concerning the rela- 

 tionships of animals as indicated by their structure and 

 developmental history. 3, Period of evolution, 1870- 

 1890, when the principle of natural selection received 

 general recognition and the zoological studies were 

 largely devoted to the applications of the theory to 

 all groups of animals. 4, Period of experimental biol- 

 ogy, since 1890, during which time have occurred the 

 remarkable advances in our knowledge of the nature of 

 organisms through the application of experimental 

 methods in the various branches of the modern science of 

 biology. 



American Zoology in 1818. 



At the beginning of the century which this volume 

 commemorates, the accumulated biological knowledge of 

 the world consisted mainly of what is to-day called 

 descriptive natural history. The zoological treatises of 

 the time were devoted to the names, distinguishing char- 

 acters and habits of the species of animals and plants 

 known to the naturalists of Europe either as native 

 species or as the results of explorations in other parts 

 of the world. This required little more than a super- 

 ficial knowledge of their general anatomical structures. 



The naturalists of those days had no conception of the 

 life within the cell which we now know to form the basis 

 of all the activities of animals and plants, nor had they 

 even the necessary means of studying such life. The 

 compound microscope, so necessary for the study of even 

 the largest of the cells of the body, was not adapted to 

 such use until 1835, although the instrument was invented 

 in the seventeenth century. With the perfection of the 

 microscope came a period of enthusiastic study of micro- 



