140 THE HISTORY OF CREATION. 



The second legion of Spongise contains the Fibrouft 

 Sponges (Fibrospongise), the soft body of which is supported 

 by a firm, fibrous skeleton. This fibrous skeleton often 

 consists merely of so-called " horny fibres," formed of a very 

 elastic, not readily destructible, organic substance. This is 

 the case for instance in our common bathing Sponge 

 (Euspongia ofiicinalis), the purified skeleton of which we 

 use every morning when washing. Blended with the 

 horny, fibrous skeleton of many of these Sponges, there 

 are numerous flinty spicula; this is the case for example 

 with the fresh -water Sponge (Spongilla). In others the 

 whole skeleton consists of only calcareous or silicious spicula 

 which are frequently interwoven into an extremely beautiful 

 lattice-work, as in the celebrated Venus' Flower Basket 

 (Euplectella). Three orders of fibrous sponges may be 

 distinguished according to the different formation of the 

 spicula, namely, Chalynthina, Geodina, and Hexactinella. 

 The natural history of the fibrous sponges is of especial 

 interest to the Theory of Descent, as was first shown by Oscar 

 Schmidt, the greatest authority on this group of animals. 

 In no other group, perhaps, can the unlimited pliability of 

 the specific form, and its relation to Adaptation and Inherit- 

 ance, be so clearly followed step by step ; perhaps in no 

 other group is the species so difiicult to limit and define. 



This proposition, which applies to the great legion of the 

 Fibrous Sponges, applies in a stiU higher degree to the 

 smaller but exceedingly interesting legion of the calcareous 

 sponges (Calcispongise), on which in 1872, after five years' 

 careful examination, I published a comprehensive Mono- 

 graph. The sixty plates of figures accompanying this Mono- 

 graph explain the extreme pliability of these small sponges, 



