4 INTRODUCTION. 



under four entirely distinct types ; namely, Vertebrata, 

 Mollusca, Articulata, and Radiata. Cuvier's system of 

 classification remained in use for many years; in fact, until 

 the progress of knowledge necessitated the adoption of a 

 better one. 



For the first time, in 1864, the attempt was made by 

 LEYDIG to grapple with the problem of the origin of the 

 Vertebrates in the light of Darwin's Theory of Evolution 

 (1858). Singular to say, although Leydig approached the 

 subject from an entirely different point of view from that 

 of Saint-Hilaire, yet he also attempted to find points of 

 affinity between the highest Insects and the Vertebrates, 

 and to identify the various subdivisions of the Vertebrate 

 brain in the brain of the bee. 



Leydig and all those later authors who would derive the 

 Vertebrates from an articulate ancestor, have started out 

 with the a priori conviction that the segmentation of the 

 body (metamerism) which is such a prominent feature (at 

 least with regard to the musculature and skeleton) in 

 fishes, and can be traced throughout the vertebrate series, 

 especially in the embryonic stages, is morphologically 

 identical with the familiar annulation or segmentation of 

 the Articulates (Annelids, Arthropods). 



This is obviously a very natural assumption to make, but 

 there is a large mass of facts which run counter to it, some 

 of which will be referred to in the following pages. 



An unexpected light was thrown upon the problem of 

 Vertebrate descent in 1866, when the Russian naturalist 

 KOWALEVSKY published an account of his researches on 

 the embryology of Amphioxus and the Ascidians. 



The Ascidians or Tunicates form a curious and in some 

 respects well-defined group of animals, which used to be 

 generally regarded as a subdivision of the Mollusca and as 



