xvi THE HISTORY OF ZOOLOGY 639 



Arthropods ; while Echinoderms are grouped with Coelenterata on 

 account of their radial symmetry, and the imperfectly understood 

 lower Worms, Sponges, and Protozoa are included in the same 

 branch. 



Cuvier may also be said to have created the science of Palaeon- 

 tology by his investigation of the Tertiary Mammalia of France. 

 As long ago as the sixth century B.C., Xenophanes had recognised 

 fossils as the actual remains of animals, but the usual view was 

 that they were merely mineral productions ; and one of the 

 earliest observers in modern times to perceive their true nature 

 was Scheuchzer, at the beginning of the eighteenth century, who 

 considered them as evidences of a universal deluge. Cuvier, as 

 well as the English geologist William Smith (1769 1839), 

 showed that the older fossils belonged to entirely different species, 

 genera, and even families, from the animals existing at the 

 present day, the differences being greater in the deeper than in 

 the more superficial formations. In this way the idea of a de- 

 finite succession of life in time was introduced. Cuvier and his 

 followers rejected, however, the notion of any genetic connection 

 between the inhabitants of successive geological periods, and 

 considered that the fauna of each epoch was exterminated by 

 some cataclysm or convulsion of nature, and the earth subse- 

 quently re-peopled by a fresh creative act. This catastrophic 

 view of the history of the earth received its death-blow in 1830- 

 33, when Sir Charles Lyell (1797- -1875) published his Principles 

 of Geology, next to the Origin of Species the most famous con- 

 tribution to natural science in modern times. By insisting on 

 the evidences for continuity in the history of the earth, he pre- 

 pared men's minds for the idea of continuity in the history of its 

 living inhabitants, and thus, more than any of the older 

 evolutionists, paved the way for the reception of Darwin's views. 



Apart from the work of Cuvier, the most important con- 

 tributions to Zoology during the first half of the nineteenth 

 century are in the domains of histology and embryology. In 

 1838 the cell-theory, according to which all parts of the body are 

 built up either of cells or of tissues derived from cells, was pro- 

 pounded first for plants by Schleiden, and shortly afterwards for 

 animals by Schwann. Both, however, had an erroneous concep- 

 tion of the cell, considering the cell-wall as its essential part- 

 whence the name celhda, a small chamber. But in 1846 the 

 " plant-slime," observed by Schleiden in the interior of the cell, 

 was investigated with great thoroughness by von Mohl and was 

 called by him protoplasm, a name originally used by Purkinje, in 

 1840, for the substance of which the youngest embryos of animals 

 are composed. Albert Kblliker and others proved that animal cells 

 existed in which no cell- wall was present, and Dujardin showed 

 that Amoebae and other lowly organisms were formed entirely of 



