A. KOWALEVSKY 269 



facts of morphology an hundredfold, may for our present 

 purpose be conveniently divided into two somewhat 

 overlapping periods, of which the second may be said to 

 begin with the enunciation by Haeckel of his Gastraea theory. 

 Within the first period fall the evolutionary speculations 

 associated with the names of Kowalevsky, Dohrn, Semper, 

 and others ; the characteristic of the second period is the 

 preponderating influence exercised upon phylogenetic 

 speculations by the germ-layer doctrine in its two main 

 evolutionary developments, the Gastraea and Ccelom 

 theories. 



In the first period we might again distinguish two main 

 tendencies, according as speculations were based mainly 

 upon anatomical or mainly upon embryological considera- 

 tions, and it so happens that these two tendencies are very 

 well illustrated by the various theories as to the origin of 

 Vertebrates which began to appear towards the 'seventies. 

 We shall accordingly, in this chapter, consider very briefly 

 the history of the earlier views on the phylogeny of the 

 vertebrate stock. 



In the early days, before the other claimants to the 

 dignity of ancestral form to the Vertebrates Balanoglossus^ 

 Nemertines and the rest had put in an appearance, there 

 were two main views on the subject, one upheld by Haeckel, 

 Kowalevsky and others, to the effect that the proximate 

 ancestor of Vertebrates was a form somewhat resembling the 

 ascidian tadpole, the other supported principally by Dohrn 

 and Semper that Vertebrates and Arthropods traced their 

 descent to a common segmented annelid or pro-annelid 

 ancestor. The former view is historically prior, and arose 

 directly out of the brilliant embryological investigations of 

 A. Kowalevsky, who proved himself to be a worthy successor 

 of the great comparative embryologist Rathke. His work 

 was indeed a true continuation of Rathke's. It was not 

 directly inspired by evolution, though it supplied much 

 useful confirmation of the theory --you may read 

 Kowalevsky's earlier memoirs and not realise that they were 

 written several years after the publication of the Origin of 

 Species. 



His first paper of evolutionary importance was a note in 



