300 THE POPULAR SCIENCE MONTHLY 



Aves, Amphibia, Pisces, Insecta and Vermes. The knowledge of this 

 last class, which included all invertebrate animals except the arthro- 

 pods, was in a very confused state, and one of the chief objects of the 

 many able zoologists of the generation immediately following him was 

 to remedy this condition. The men whose services were greatest in this 

 direction were 0. F. Mliller, Lamarck and Cuvier. In 1794 Lamaick 

 first distinguished the vertebrates from the invertebrates and sub- 

 divided the latter group into the five classes of Mollusca, Insecta, 

 Vermes, Echinodermata and Polypi. Thus a long step was taken 

 towards modernizing the system and this early effort of Lamarck may 

 be said to be the first modern classification of animals. He, in his 

 later works, further subdivided the invertebrate types until he had ten, 

 the fundamental idea at the basis of his classification being that the 

 various groups of animals constitute a single ascending series which 

 begins with the lowest and ends with the highest. This principle of 

 the unity of the type found a wide acceptance among the naturalists 

 of that time and was based upon the law : Natura non f acit saltum. 



In 1812 Cuvier published his division of the animal kingdom into 

 four branches or types and in 1817 his great work " Le Eegne Animal " 

 which established the second great reform of the system and was des- 

 tined to exert an influence only second to that of Linngeus's " Systema 

 Naturae " upon the study of animals and the development of the sys- 

 tem. In these works Cuvier controverted the principle of the unity of 

 iy^e. among animals and taught that instead of one four distinct and 

 permanent types prevail. It was upon these four types that he based 

 his four fundamental branches of the animal kingdom : A^ertebrata, 

 Articulata, Mollusca and Zoophyta or Eadiata. 



A comparison of this classification with that of Linnasus will show 

 what a tremendous advance had been made in the development of the 

 system in the half century separating them. The group of animals 

 which had benefited most in this general advance was probably the Mol- 

 lusca, which was Cuvier's special field of research. The loAvest group 

 in Cuvier's system, as that in Linngeus's, was the one about which the 

 least was known, the Zoophyta or Eadiata being made up of several dis- 

 tinct and heterogeneous groups of animals which bore no near relation- 

 ships to one another. 



This condition led to an active investigation during the generation 

 immediately following of all the lower animals and a A^ery large num- 

 ber of Avorks of fundamental importance appeared. Eudolphi studied 

 the parasite worms, Tiedemann and L. Agassiz the anatomy and 

 Johannes Miiller the development of echinoderms, Ehrenberg the 

 microscopic animals, Eschscholtz, Sars and others jellyfish and polyps. 

 The knowledge of these two latter groups was also very much extended 

 as the result of various scientific expeditions which were sent out by 



