xvi THE HISTORY OF ZOOLOGY 667 



has been completely revolutionised. Specially remarkable is the 

 advance in our knowledge ofxthe Protozoa, Sponges, Actinozoa, 

 Echinoderms and Amphioxus. The new light which has been 

 thrown on the affinities of Balanoglossus, Rhabdopleura, and 

 Cephalodiscus is also worthy of special mention. Probably the 

 greatest of comparatively recent embryological triumphs, belonging 

 to the earlier part of the period now under discussion, is Kowalew- 

 sky 's discovery of the notochord and hollow nervous system of 

 the Tunicate larva, which resulted in the removal of the Urochorda 

 from Molluscoida to Chordata, and in breaking down the sharp 

 line between Vertebrates and Invertebrates. 



But perhaps the most remarkable result of improved micro- 

 scopical technique is the rise and development of a distinct 

 department of histology, known as cytology, dealing with the 

 minute structure of the protoplasm and nuclei and the various 

 intra-cellular phenomena, such as mitosis. Our knowledge of this 

 subject is entirely a product of the last forty-five years, and is 

 due in great measure, in the first instance, to the researches of 

 W. Fleming, E. Strasburger, and E. van Beneden. A 

 modification of the cell-theory has also been necessitated by the 

 proof that many animal tissues do not consist of distinct cells, 

 but of a continuous mass of protoplasm with more or less regularly 

 arranged nuclei, and are therefore strictly not multicellular but 

 non-cellular. As certain Protozoa, such as the Mycetozoa and 

 Opalina, are also non-cellular, containing numerous nuclei in an 

 undivided mass of protoplasm, the distinction between Protozoa 

 and Metazoa appears to be less absolute than it was at one time 

 considered. 



The advance in palaeontology during the same period has also 

 been immense. In particular the researches of E. D. Cope, 

 O. C. Marsh, and others in America, have added whole orders to 

 Zoology the Odontolcae, Ichthyornithes, Stereornithes, Ambly- 

 poda and Dinocerata and have resulted in the discovery of many 

 new and strange forms among the Dinosauria, Elasmobranchs, 

 Ganoids, and other groups, and in the tracing of the pedigrees 

 of the Equidae, Camelidae, and other Mammalian families. Im- 

 portant though less striking discoveries have also been made 

 among the fossil faunae of Europe, India, South Africa, and 

 Australia ; while among Invertebrates the attempts to trace the 

 pedigree of the Ammonites and Brachiopods are specially note- 

 worthy. 



In embryology an important landmark is furnished by F. M. 

 Balfour's Comparative Embryology (1880-81) ; in distribution, by 

 A. R. Wallace's Geographical Distribution of Animals (1876), 

 each the first complete treatise on the subject in question. The 

 zoo-geographical regions adopted by Wallace were originally 

 proposed by P. L. Sclater in 1857. Similar landmarks for Zoology 



