j6 HUXLEY 



of their evolution from those lower organisms. It is 

 easy enough to us, looking back, to see what a key to 

 the proof of the fundamental unity of living things 

 this supplied ; but even the prevision of Huxley, 

 shown so markedly in many ways, was obscured by 

 the dominance of the notion of fixity outside certain 

 well-marked lines. For in 1853 ne writes that " there 

 is no progression from a lower to a higher type, but 

 merely a more or less complete evolution of one 

 type." Nevertheless, his acute comparison between 

 the Coelenterata and the Coelomata was destined to 

 supply proof of the progression which he questioned. 

 His discovery, says Professor Allman, 



that the body of the Medusae is essentially composed 

 of two membranes, an outer and an inner, and his 

 recognition of these as the homologues of the two 

 primary germinal leaflets in the vertebrate embryo, is 

 one of the greatest claims of his splendid work on the 

 recognition of ^zoologists. This discovery stands at 

 the very base of a philosophical zoology, and of a 

 true conception of the affinities of animals. It is the 

 ground on which Haeckel has founded his famous 

 Gastraea-theory, and without it Kowalesky could never 

 have announced his great discovery of the affinity of 

 the Ascidians and Vertebrates, by which zoologists 

 have been startled. 1 



Noting, by the way, that before Huxley sailed in 

 the Rattlesnake, he had made the interesting discovery 



1 1. 40. 



