34 The Science of Life. 



in the Encyclopedia Britannica, from the introduction 

 to his Anatomy of the Invertebrates (1870), from the 

 introduction to zoology, entitled The Crayfish (1881), 

 from Man's Place in Nature (1863), and American 

 Addresses (1879). 



His more technical scientific memoirs are being re- 

 published under the editorship of Profs. Michael Foster 

 and E. Ray Lankester, and among the most important 

 may be noticed those which discuss the anatomy and 

 affinities of the Medusae (1849) (whence sprang the 

 generalization that the embryonic epiblast and hypo- 

 blast correspond to the two layers of a polype's body), 

 the fossil ganoids, the vertebrate skull (including an 

 anatomical demolition of the vertebral theory which 

 lasted from Oken to Owen), the classification of birds 

 (based on the skeletal features of the skull), the union of 

 birds and reptiles in the major group Sauropsida, and of 

 amphibians and fishes in the major group Ichthyopsida. 



Two great biological books were completed in 1866, 

 Mr. Herbert Spencer's Principles of Biology and Prof. 

 Ernst Hseckel's Generelle Morphologie ; and 

 though they are very different in mood and 

 style, they have the common aim of presenting an 

 ordered system of biological generalities. In the 

 Generelle Morphologie, we find long discussions on the 

 forms assumed by organic structures and by entire 

 organisms, a subject (" promorphology") to which little 

 attention has been paid since ; on the theory and grades 

 of individuality both physiological and morphological, 

 a subject which was pursued by many till all biologists 

 wearied of it; on the categories of homology and the 

 principles of classification; on the different modes of 

 reproduction ; on heredity and evolution. Like its 

 English analogue mentioned above, it presented not 

 only a critical account of the general conclusions which 

 had been reached, but a further development of each, and 

 an orderly arrangement of the whole. To those who 

 seek for a survey of the whole field in the perspective of 

 1866, which has not been essentially changed since, the 

 two works are invaluable, as also to those who fancy 

 that they have new ideas on the subject. 



