56 DEGENERA TION. 



hypothesis of degeneration, even in the absence of any 

 confirmatory evidence from embryology. When we 

 so apply this hypothesis we find in not a few cases, in 

 working over the details of the organization of man}^ 

 different animals by the light which it affords — that 

 much becomes clear and assignable to cause which, 

 on the hypothesis either of "balance" or of " elabora- 

 tion," is quite hopelessly obscure. As examples of 

 groups of animals which can thus be satisfactorily 

 explained I may cite first of all the Sponges : as only 

 somewhat less degenerate, we have all the Polyps 

 and Coral-animals, also the Starfishes. Amongst 

 the Mollusca — the group of headless bivalves, the 

 oysters, mussels and clams, known as the Lamelli- 

 branchs, are, when one once looks at their structure 

 in this light, clearly enough explained as degenerated 

 from a higher type of head-bearing active creatures 

 like the Cuttle-fish ; whilst the Polyzoa or Moss- 

 polyps stand in precisely the same kind of relation 

 to the higher Mollusca as do the Ascidians to the 

 higher Vertebrates : they have greatly degenerated, 

 and become minute encrusting organisms which, like 

 some of the Ascidians, build up colonies by plant-like 

 budding growth. Tlie Rotifers, or wheef animalcules. 



