658 ZOOLOGY 



SECT. 



primary sub-division, and the articulated worms associated with 

 Arthropods ; while Echinoderms are grouped with Ccelenterata 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 Palae- 

 ontology by his investigations 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 definite succession 

 of life in time was introduced. Cuvier and his followers rejected, 

 however, the notion of any genetic connection between the in- 

 habitants of successive geological periods, and considered that the 

 fauna of each epoch was exterminated by some cataclysm or 

 convulsion of nature, and the earth subsequently 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 contribution to natural 

 science in modern times. By insisting on the evidences for con- 

 tinuity in the history of the earth, he prepared 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 contri- 

 butions 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 propounded 

 first for plants by Schleiden and shortly afterwards for animals 

 by Schwann. Both, however, had an erroneous conception of 

 the cell, considering the cell-wall as its essential part whence 

 the name cellula, a small chamber. But in 1846 the " plant- 

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

 vestigated 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 animaJs are 

 composed. Albert Kolliker and others proved that animal-cells 

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



