INTRODUCTION. Ixv 



authorized to give the name of protoplasm to the semi-fluid substance, azotic, made 

 yellow by iodine, whicli is spread throughout the cellular cavity, and which furnishes 

 the material for the primordial utricle and nucleus." Thus a comparative anatomist 

 and a botanist each independently applied the same name to the living substance com- 

 mon to both animals and plants. 



Dujardin's researches attracted great attention, and in 1861 Max Schultze did not 

 hesitate to affirm the identity of animal cells in general with sarcode. Brucke (1861), 

 Schultze (1863), and Kilhne (1864), finally demonstrated the identity of living mat- 

 ter in the two kingdoms, as to its fundamental physical properties : irritability and 

 contractility. 



Since the beginning of the second half of this century (1850-1884), zoological 

 science has been developed with great rapidity in all directions, but in none more than 

 in embryology and morphology, v/hile the number of workers lias vastly increased. 

 Moreover, the general respect and sympathy for biological research, felt by all edu- 

 cated minds, has encouraged the active workers ; hence the formation and endowment 

 of new academies and societies, the establishment and support of journals of advanced 

 m ^rjjhology ; the building and rearrangement of museums, and the installation of labor- 

 atories for original research. Private explorations in all parts of the earth, and the 

 numerous surveys, especially those of the United States, both state and national, have 

 fostered and extended zoological knowledge. 



Several general treatises on embryology have been published, l)y Wolff (1759), Von 

 Baer, Agassiz, Haeckel and Packard, but the last of these treatises, Balfour's Compar- 

 ative Emljryology, is an epoch-making work on development, indicating the third 

 stage in the history of biological science ; Wolff's marking the first, and Von Baer's 

 the second. 



The great steps in the discovery of the way animals reproduce and develop were 

 the discovery of spermatozoa by Leeuwenhoeck in 1677, and that of the mammalian 

 egg by Degraaf in 1673. A century and a half later Von Baer confirmed the latter, 

 and showed that all mammals develop from eggs, and then Coste, Valentin, and Jones 

 showed that these eggs were hornologous with those of the lower vertebr.ates. 

 The next step was the discovery by Remak, in 1850, of the three germinal layers; 

 then Huxley, in 1859, homologized these with the tissues of the cffilenterates. The 

 last steps to be mentioned are investigations of the brothers Hertwig on the meso- 

 blast and the coelom, and those of Lang and of Sedgwick on metameric segmentation 

 and the homology of the blastopore throughout the animal kingdom. While the 

 future will doubtless produce many important discoveries, and corrections of existing 

 errors, it would seem that the leading features of embryology are already estal)lished. 



The earlier writers on evolution were Lamarck, Geoffrey St. Hilaire, and Goethe. 

 The literature of evolution, which has characterized the second half of this century, 

 is the scientific oft"spnng of Darwin's Origin of Species, which appeared in 1859, a pre- 

 liminary essay by Darwin and by Wallace being offered the previous year. It is the 

 leaven which has leavened the whole lump of modern scientific and philosophical 

 thought. It was the work of a zoologist, whose studies of systematic and anatomical 

 zoology as well as geographical distribution converged toward the conception that 

 sjiecies had originated from natural causes. Alfred R. Wallace, also, as the result of 

 his travels and researches on the Amazon River, and especially in the Malay archipel- 

 ago, arrived nearly simultaneously at the same conclusion ; his original essay written 

 at Sarawak iu 1855, with others, collected iu 1870, are entitled Contributions to the 



