458 ON CONCRETIONS, SPICULES, ETC. [ch. 



in most animals, a local predominance of one particular species. 

 On the contrary, in a little pinch of deep-sea mud or of some fossil 

 " Radiolarian earth," we shall probably find scores, and it may be 

 even hundreds, of different forms. Moreover, the radiolarian 

 skeletons are of quite extraordinary delicacy and complexity, in 

 spite of their minuteness and the comparative simphcity of the 

 "unicellular" organisms within which they grow; and these 

 complex conformations have a wonderful and unusual appearance 

 of geometric regularity. All these general considerations seem 

 such as to prepare us for the special need of some physical 

 hypothesis of causation. The little skeletal fabrics remind us of 

 such objects as snow-crystals (themselves almost endless in their 

 diversity), rather than of a collection of distinct animals, con- 

 structed in apparent accordance with functional needs, and dis- 

 tributed in accordance with their fitness for particular situations. 

 Nevertheless great efforts have been made of recent years to 

 attach "a biological meaning" to these elaborate structures; 

 and "to justify the hope that in time the utilitarian character 

 [of the skeleton] will be more completely recognised*." 



In the majority of cases, the skeleton of the Radiolaria is 

 composed, like that of so many sponges, of silica ; in one large 

 family, the Acantharia (and perhaps in some others), it is composed, 

 in great part at least, of a very unusual constituent, namely 

 strontium sulphate f. There is no fundamental or important 

 morphological character in which the shells formed of these two 

 constituents differ from one another; and in no case can the 

 chemical properties of these inorganic materials be said to influence 

 the form of the complex skeleton or shell, save only in this general 

 way that, by their rigidity and toughness, they may give rise to 

 a fabric far more delicate and slender than we find developed 

 among calcareous organisms. 



A slight exception to this rule is found in the presence of true 

 crystals, which occur within the central capsules of certain Radio- 



* Cf. Gamble, Radiolaria (Lankester's Treatise on Zoology), vol. i, p. 131, 1909. 

 Cf. also papers by V Hacker, in Jen. Zeitschr. xxxix, p. 581, 1905, Z. f. wiss. 

 Zool. Lxxxra, p. 336, 1905, Arch. f. Protistenkunde, ix, p. 139, 1907, etc. 



f Biitschli, Ueber die chemische Natur der Skeletsubstanz der Acantharia, 

 Zool. Anz. »xx, p. 784, 1906. 



