243 ERNST HAECKEL. 



conceived in a genealogical sense as stems or phyla, but that 

 the higher phyla of the animal kingdom (Vertebrata, Mol- 

 lusca, Arthrojjoda, Echinodermata) are to be considered as 

 diverging descendants from the lower stems of the Vermes, 

 which have taken their origin from diverse branches of this 

 numerous lower animal group ; and that, lastly, the 

 Vermes and the Ccelenterata must have started off from those 

 still lower groups of organisms, the Protozoa or Protista (1. c., 

 j)p. 413, 414). I have more definitively expressed this 

 opinion in the first edition of my ' Naturliche Schopfungs- 

 geschichte ' (1868), and in the succeeding editions I have 

 sought to state it more clearly. I failed in evolving it into 

 perfect clearness, because the Gastraea-theory, to which I first 

 of all was led by my ' Monograph of Calcareous Sponges,' was 

 not yet formed. It was only by means of the Gastraea-theory 

 and its consequences that the phylogenetic relationship of 

 the types of animals to one another was completely cleared up. 



It might be asserted that the Gastraea-theory is only a 

 reform or modification of the theory of types, because three 

 of the primitive four types (Vertebrata, MoUusca, and Arthro- 

 poda) have been retained nearly within the original limits of 

 their conception, but the content of this conception has 

 become completely difierent. Besides, moreover, between 

 the two theories there is this most essential difference, that 

 in the " type-theory " the types aj)pear as co-ordinate, self- 

 existing groups of forms of equal morphological value, along- 

 side each other, and yetindependent one of another; whereas 

 in the " Gastraea-theory" the phyla exist as partly co-ordi- 

 nate, partly subordinate, groups of completely different 

 morphological value ; partly near, partly alongside each other, 

 but all in a common connection. 



In the excellent explanation which Gegenbaur has given 

 in the second edition of his ' Grundziige der vergleichenden 

 Anatomic' (1872, p. 72) of the animal types, these various 

 references of types of different value to one another have 

 been clearly explained, and, through the most sagacious 

 investigation of details, has been further strongly built on 

 the sure foundation of comparative anatomy. Gegenbaur 

 shows that the seven types or phyla have their limits some- 

 times tolerably distinctly fixed, and sometimes are by no 

 means to be distinguished from one another ; that one must 

 distinguish between the lower and higher types, and that the 

 different higher types or phyla disclose in their common 

 point of departure the lower. Through this demonstrable 

 connection of the phyla it will appear that the whole of the 

 members of the animal kingdom can be placed in a near 



