682 ZOOLOGY sect. 



Protozoa to Vermes ; Vaughan Thomson defined the Polyzoa, 

 and Rudolphi, Leuckart, and von Siebold showed that the 

 Flat-worms were in no sense Zoophytes. Sponges were con- 

 sidered by some as polypes, by others as plants ; the current of 

 water flowing in at the pores and out at the oscula was discovered 

 by Robert Grant about 1820 : later Bowerbank demonstrated 

 the presence of cilia, and the full proof of their animal nature was 

 made by the researches of LieberkUhn and Carter. The Fora- 

 minifera were classed as Cejjhalopoda until the 'thirties, when 

 Dujardin determined their proper place by the discovery of the 

 living protoplasmic body. Other important advances in classifica- 

 tion were the separation of Cirripedia from Mollusca by Vaughan 

 Thomson, and the withdrawal from intestinal worms of the 

 parasitic Copepoda and of the Pentastomida. The Infusoria 

 have also had a chequered history. Ehrenberg in his magni- 

 ficent work Die Infusionsthiere, looked upon the food-vacuoles as 

 stomachs, and described a complex enteric canal connecting them ; 

 it is, therefore, not surprising that he considered them as belong- 

 ing to the same group as Rotifers. Louis Agassiz, as late as 

 1859, considered Paramoecium, Opalina, &c., to be the young of 

 Planarians and Trematodes and Vorticella to be a Polyzoan, and 

 it was only by the researches of Stein and others that the class of 

 Infusoria was fully established as a natural group of unicellular 

 organisms. 



The Swiss zoologist, Agassiz (1807-73), referred to in the 

 preceding paragraph, is interesting, not only as one of the foremost 

 naturalists of his time and the founder of the large and active 

 school of zoologists in the United States, where he spent the 

 latter part of his life, but also as the last great biologist to 

 maintain the fixity of species. In his Essay on Classification, 

 published, curiously enough, in the same year (1859) as the 

 Origin of Species, he supports the proposition that the various 

 subordinate groups of animals, from phyla to species, are not 

 mere " devices of the human mind to classify and arrange our 

 knowledge in such a manner as to bring it more readily within 

 our grasp and facilitate further investigations," but that they 

 " have been instituted by the Divine Intelligence as the categories 

 of His mode of thinking." In other words, that in our classifica- 

 tions we " have followed only, and reproduced, in our imperfect 

 expressions, the plan whose foundations were laid in the dawn of 

 creation." 



In 1859 occurred what may fairly be called the most important 

 event in the history of biological science, the publication of 

 Charles Darwin's Origin of Species. The evolutionary theories 

 of Buffon, Erasmus Darwin, Lamarck, and Geoffroy St. Hilaire 

 had produced little effect upon contemporary zoology; and 

 Robert Chambers's Vestiges of Creation (1844), although exciting 



