xvi THE HISTORY OF ZOOLOGY 661 



and Rudolphi, Leuckart, and von Siebold showed that the 

 Flat-worms were in no sense Zpophytes. Sponges were considered 

 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 Lieberkiihn and Carter. The Fora- 

 minifera were classed as Cephalopoda until the 'thirties, when 

 Dujardin determined their proper place by the discovery of the 

 living protoplasmic body. Other important advances in classi- 

 fication 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 magnificent 

 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 belonging to 

 the same group as Rotifers. Louis Agassiz, as late as 1859, 

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

 arians 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 pre- 

 ceding 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 investi- 

 gations," but that they " have been instituted by the Divine 

 Intelligence as the categories of His mode of thinking." In other 

 words, that in our classifications we " have followed only, and re- 

 produced, 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 Geoffrey St. Hilaire 

 had produced little effect upon contemporary zoology ; and 

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

 great interest, was too crude and speculative to make many con- 

 verts among men of science. But Darwin had the advantage of 

 being not only a philosopher, but also a naturalist in the broadest 



