140 THE HISTORY OF CREATION, 
The second legion of Spongiz contains the Fibrous 
Sponges (Fibrospongiz), 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 officinalis), 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 difficult to limit and define. 
This proposition, which applies to the great legion of the 
Fibrous Sponges, applies in a still higher degree to the. 
smaller but exceedingly interesting legion of the calcareous 
sponges (Calcispongize), 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 
