202 Dr. TVallich on the Origin of the 



sponges occupying the area in which they flourish in the 

 highest degree, owing to the uniformity of all the above cou- 

 ditionSj would proceed pari passu. It would follow, there- 

 fore, as a natural consequence, that the time requisite for the 

 growth and arrival at maturity of the whole series would, in 

 like manner, become uniform. The uniformity, moreover, of 

 the supply of food, inseparable from the nature of the case — 

 nay, the physical necessity that in a vast fluid medium like 

 the ocean difi'usion would take place with unerring unifor- 

 mity of all the inorganic and organic substances on which 

 nutrition depends — would assist, if not actually enforce, a rate 

 of growth uniform in the groups distributed over the same 

 areas. And thus the various groups would necessarily arrive 

 simultaneously at that stage of their being when their asphyx- 

 iation by the supercharging of their protoplasmic masses with 

 silica would end their career. 



If we reflect, moreover, that we are dealing with conditions 

 that must have been equally real and effective ever since the 

 period when the earliest flint-producing deposits began to be 

 formed at all, we can hardly doubt that the law which governs 

 the growth of the sponges at the bottom of the deep sea 

 must have caused them to complete their first cycle in the 

 history of the flint-bearing chalk within a certain cosmic 

 period, and that, owing to the uniformity of the conditions 

 which have ever since prevailed, there must have been an 

 approximate uniformity in the completion of each cycle since 

 that period. 



As the result of these operations, extensive areas of the cal- 

 careous sea-bed would, after a certain period, be simultaneously 

 covered with protoplasm supersaturated with silica in its 

 gelatinous condition, and a constant coalescence and tearing 

 asunder of portions of the masses would take place, owing 

 partly to their inherent contractility and diminution in volume 

 through the expulsion of their combined water. Judging 

 from what is known of the time necessary to bring about the 

 change in silica from the gelatinous to the nearly perfectly 

 anhydrous state, when it may be said to become finally conso- 

 lidated into a hard, stony mass, it is not improbable that the 

 process would not be a very protracted one, even when con- 

 ducted on the vast scale referred to — a fact, if it be one, which 

 would materially decrease the possibility of the extinction of 

 the minute forms of life that build up the calcareous deposits. 

 They would perpetuate their species in the intervals unoccu- 

 pied by the nascent flint-masses, and gradually entomb them. 

 On the other hand, the sponges would perpetuate their species 

 by gemmules distributed over the general surface of the sea- 



