68 



ESSAY ON CLASSIFICATION. 



Part I. 



generalizations respecting the mode of formation of animals ; for he first discovered, 

 in 1827, the ovarian egg of Mammalia, and thus showed for the first time, that 

 there is no essential difference in the mode of reproduction of the so-called vivip- 

 arous and oviparous animals, and that man himself is developed in the same manner 

 as the animals. The universal presence of eggs in all animals and the unity of their 

 structure, which was soon afterwards fully ascertained, constitute, in my opinion, the 

 greatest discovery of modern times in the natural sciences.^ 



It was, indeed, a gigantic step to demonstrate such an identity in the material 

 basis of the development of aU animals, when their anatomical structure was already 

 known to exhibit such radically different plans m their full-grown state. From that 

 time a more and more extensive investigation of the manner in which the first 

 germ is formed in these eggs, and the embryo develops itself; how its organs 

 grow graduall}^ out of a homogeneous mass; what changes, what complications, what 

 connections, what functions they exhibit at every stage ; how in the end the young 

 animal assumes its final form and structure, and becomes a new, independent being, 

 could not fail to be the most interesting subject of inquiry. To ascertain all this, 

 in as many animals as j^ossible, belonging to the most different types of the animal 

 kingdom, became soon the principal aim of all embryological investigations ; and it 

 can truly be said, that few sciences have advanced with such astonishing rapidity, 

 and led to more satisfactory resulis. 



For the actual phases of the mode of development of the different types of the 

 animal kingdom, I must refer to the special works upon this sulyect,^ no general 



zig, 1839-42, 2 vols. 8vo. — Valentin, (G.,) Hand- 

 bucli der Entwickolungsgesehichte, etc., Berlin, 1835, 

 1 vol. 8vo. — Lulubuch der Physiologic dcs Men- 

 sclien, Braunschweig, 1843, 2 vols. 8vo. — Longet, 

 (F. A.,) Traite de Physiologie, Paris, 1850, 2 vols. 

 8vo. — KoLLiKER, (Alb.,) Microscopische Anatomic 

 dcs Menschen, Leipzig, 1840-54, 2 vols. 8vo. fig. — 

 See also Owen's Lectures, etc., Siebold und Stan- 

 NlDs's Lehrbuch, and Carus's Morphologic, q. a. 

 p. 27, and p. 18. I might further quote almost every 

 modern text-book on physiology, but most of them 

 are so evidently mere compilations, exhibiting no 

 acquaintance with the subject, that I omit purposely 

 to mention any other elementary works. 



' Baer, (C. E. a,) De Ovi Mammaliuni et 

 Hominis Genesi, Konigsberg, 1827, 4to., fig. — 

 PuRKiNJE, (.J. E.) Symbolte ad ovi avium historiam 

 ante incubationem, Lipsi.-c, 1830, 4to. fig. — Wag- 



ner, (E.,) Prodromus Historiao gencrationis Hominis 

 atque Anim.ilium, etc., Lipsiie, 183G, 1 vol., fol., fig. 

 — Icones physiohjgicas Lipsiie, 1839, 4to. fig. 



^ The limited attention, thus far paid in this 

 country to the study of Embryology, has induced 

 me to enumerate more fully the works relating to 

 this branch of science, than any others, in the hope 

 of stimulathig investigations in that direction. There 

 exist upon this continent a number of types of ani- 

 mals, the embryological illustration of which would 

 add immensely to the stock of our science ; such 

 are the Opossum, the Ichthyoid Batrachians, the 

 Lepidosteus, the Aniia, etc., not to speak of the 

 opportunities which thousands of miles of sea-coast, 

 everywhere easily accessible, alTord for embryologi- 

 cal investigations, from the borders of the Arctics 

 to the Tropics. In connection with Embryology 

 the question of Individuality comes up naturally. 



