136 DR. J. S. BOWERBANK ON SPONGES. [Feb. 13, 



The nature of these characters is not that of organisms indis- 

 pensably necessary to the existence of a sponge, but rather of those 

 of an auxihary description, which one species may require while 

 another may well dispense with them. 



As our knowledge of species extends, we quickly find that there 

 are groups existing among them marked by peculiar modifications 

 of the more essential organs of the sponge, such as those of the 

 various modes of arrangement of the mechanical structure of the 

 skeleton, while the material of which it is formed remains the same 

 through a considerable number of such groups. Thus we arrive by 

 a natural process at generic groups, which are on this constructive 

 anatomical system quite as natural as species. Hitherto we have 

 had but few well-defined genera of this description ; and these few have 

 been, as it were, thrust upon us by the irresistible evidence of Nature 

 herself. Contrary to the received opinion of many able naturalists, 

 I firmly believe that genera are quite as natural as species, and that 

 it only requires perseverance on our own parts to recognize them, 

 and to define their boundaries with accuracy and certainty. Of 

 course we must expect cases occasionally to arise in which they ap- 

 proach each other by such insensible degrees as to render it extremely 

 difficult to distinguish between the approximating points ; but this 

 appears also to be the case in botany, and in other branches of 

 natural history, and may be considered the exception and not the 

 rule of nature. Having thus established a natural foundation to 

 our system of genera, the next step is that of classification ; and 

 here the material of which the skeleton is formed becomes a most 

 important point of consideration. We find hi the higher classes in 

 zoology the animals of the highest degrees of organization secreting 

 phosphate of lime as the basis of the skeleton ; and as we descend 

 in the scale of creation, the phosphate is replaced by carbonate of 

 lime, and the skeleton becomes external instead of internal ; the 

 next step downward, and we lose the earthy material, and the kera- 

 tode skeleton supplies its place; membrane succeeds to keratode ; and 

 finally all these substances are wanting, and sarcode becomes the 

 entire anim.al. But amidst all these changes we find no class of ani- 

 mals secreting silex as the basis of its skeleton ; this material is re- 

 served as the great distinguishing character of the class Protozoa. 



Among the Spongiadse we observe a natural simulation, as it were, 

 of this gradational retrogression of organization. None of them 

 secrete phosphate of lime ; but the highest organized species secrete 

 carbonate of lime in abundance. The next gradation is the secretion 

 of silex in place of lime, the silex being the predominant material of 

 the skeleton ; and to this tribe belong by far the greater number of 

 existing sponges. Then follows an intermediate stage, similar to that 

 of the cartilaginous fishes, where we find the cartilage the predomi- 

 nant material, with included floating or dispersed portions of earthy 

 basis ; so in the sponges, as in Chalina and other genera, we have kera- 

 tode the essential portion of the skeleton, with siliceous spicula im- 

 mersed in the fibres to give additional strength and substance to the 

 Structures ; and, finally, the skeleton becomes pure keratode, without 



