I A CHAPTER IN DARWINISM 45 



nutrition, or again, by minute size of its representatives 

 there we are justified in applying the 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 organisation of many 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 "elaboration," 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 Star- 

 fishes. Amongst the Mollusca the group of head- 

 less bivalves, the oysters, mussels and clams, known 

 as the Lamellibranchs, 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 l 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. The Eotifers, or wheel animalcules, 

 I have already mentioned as best explained by the 

 supposition that they are the descendants of far larger 

 and more fully -organised animals provided with loco- 



1 Not to the higher Mollusca in all probability, but to some 

 higher worm-like form. The ancestry of the Polyzoa is still a pro- 

 found mystery ! December, 1889. 



