V WE ISMANN'S EXPLANATION 207 



and thus, to my mind at least, the general validity of Weis- 

 mann's conclusion falls to the ground. 



' That this conclusion — putting aside the fact that it does 

 not regard the ultimate causes of the degenerations — is fully 

 justified with respect to a large number of characters is, 

 for every one who accepts the principle of utility, a fact 

 long established and indisputable. Its justice is proved most 

 clearly and most simply, for instance, by the disappearance of 

 the adapted colouring of the wild ancestors in our domestic- 

 ated animals — e.g. rabbits — in consequence of domestication. 



But the conclusion does not hold good for any indifferent 

 characters, including those which depend on correlation, nor 

 for those which are accidentally useful and which have arisen 

 from and are maintained by external influences. 



That indifferent structures may undergo degeneration 

 appears from physiological considerations indisputable. In 

 his short paper, "The Origin of New Species through the 

 Decay and Disappearance of Old Characteristics," Oscar 

 Schmidt has, as I have already remarked, pointed out a case 

 in sponges which, in my view, bears upon this subject. It 

 concerns the genus Caminus. 0. Schmidt had previously 

 said that the fine Caminus Vulcani of the Adriatic Sea 

 probably belonged to the Tetractinellidae, notwithstanding the 

 absence in it of the four-rayed siliceous spicules characteristic 

 of that order. Subsequently he obtained a Caminus (C. 

 osculosus, Grube) which contained in no small numbers such 

 spicules in process of degeneration. 



In the specimen which Grube had before him, however, 

 these spicules, as could be still shown, were very rare, and 

 in some preparations entirely wanting. On re-investigating 

 Caminus Vulcani, 0. Schmidt found in this sponge also 

 scattered remains of such degenerate four-rayed spicules, but 

 none in Caminus apiarium. " And thus," says 0. Schmidt, 

 " in Caminus proof is afforded that by the disappearance of an 



