668 ZOOLOGY sue*. 



as a whole are Huxley's Anatomy of Vertebrated Animals (1871) 

 and Anatomy of Invertebrated Animals (1877), Carl Gegenbaur's 

 Elements of Comparative Anatomy (English edition, 1878), Glaus 's 

 Text-Book of Zoology (1st English edition, 1884-5), Ray 

 Lankester's Notes on Embryology and Classification (1877), and the 

 same author's articles in the Encyclopedia Britannica (9th edition). 

 Both Glaus and Gegenbaur retain Vermes as a primary division ; 

 Lankester was the first to split up that unnatural assemblage into 

 distinct phyla, and to include Balanoglossus and the Tunicata 

 among Vertebrates, and Xiphosura and Eurypterida among 

 Arachnida. He also associated Rotifers and Chsetopods with 

 Arthropoda, and placed Hirudinea among the Platyhelminthes. 

 A later development of the same author's views on morphology 

 and classification is embodied in his Treatise on Zoology, of which 

 eight volumes have now been published (see Appendix, 675). Of 

 inestimable value in the advancement of the embryology of Verte- 

 brates is the comprehensive Handbuch (1901-1906) of O. Hertwig, 

 with sections by various other embryologists. 



The student who is interested in the permutations and combina- 

 tions of modern classification may be referred to the works just 

 quoted as well as to the numerous text-books published of late 

 years. The most important point to notice in this connection 

 is the breaking down of the sharp boundaries between the four 

 Cuvierian Branches and a return to something like the conception 

 of unity of type, expressed, however, not as a linear series, but 

 as a branch-work with the most complex and often puzzling inter- 

 relations. 



Among the numerous recent contributions to philosophical 

 Zoology it must suffice to mention the works on heredity and 

 kindred subjects of August Weismann, the most prominent 

 member of the ultra-Darwinian school, who deny use-inheritance 

 and rely upon natural selection as the main, if not the sole, factor 

 in evolution. The opposite view, which accepts the truth of use- 

 inheritance, is mainly supported by the American school of Neo- 

 Lamarckians. Weismann has also resuscitated the theory of 

 preformation under a modern form. He considers that the 

 various parts of the adult organism are represented in the 

 chromatin (germ-plasm) of the sex-cells by ultra-microscopic 

 particles or determinants. These and allied topics are comprehen- 

 sively treated from a different standpoint by O. Hertwig in his 

 Allgemeine Biologic (1909). 



In a brief sketch like the present it is impossible to do more 

 than refer, in general terms and without mention of names, to the 

 vast amount of work now being done in every department of 

 Zoology. The output of original research is greater than at any 

 former time and is increasing rapidly, and every important addition 

 to our knowledge necessitates a more or less thorough reconsidera- 



