466 ON CONCRETIONS, SPICULES, [ch. 



surface-energies, and also doubtless with difEerences in the total 

 available amount of surface- energy in relation to gravity or other 

 extraneous forces. In my early student days, Wyville Thomson 

 used to tell us that certain deep-sea "Difflugias," after constructing 

 a shell out of particles of the black volcanic sand common in parts 

 of the North Atlantic, finished it off with "a clean white collar" 

 of little grains of quartz. Even this phenomenon may be accounted 

 for on surface-tension principles, if we assume that the surface- 

 energy ratios have tended to change, either with the growth of 

 the protoplasm or by reason of external variation of temperature 

 or the like; and we are by no means obliged to attribute the 

 phenomenon to a manifestation of voUtion, or taste, or aesthetic 

 skill, on the part of the microscopic organism. Nor, when certain 

 Radiolaria tend more than others to attract into their own sub- 

 stance diatoms and such-like foreign bodies, is it scientifically 

 correct to speak, as some text-books do, of species "in which 

 diatom selection has become a regular habit.'''' To do so is an 

 exaggerated misuse of anthropomorphic phraseology. 



The formation of an "agglutinated" shell is thus seen to be 

 a purely physical phenomenon, and indeed a special case of a 

 more general physical phenomenon which has many other 

 important consequences in biology. For the shell to assume the 

 solid and permanent character which it acquires, for instance, in 

 Difflugia, we have only to make the additional assumption that 

 some small quantities of a cementing substance are secreted by 

 the animal, and that this substance flows or creeps by capillary 

 attraction between all the interstices of the little quartz grains, 

 and ends by binding them all firmly together. Rhumbler* has 

 shewn us how these agglutinated tests, of spicules or of sand- 

 grains, can be precisely imitated, and how they are formed with 

 greater or less ease, and greater or less rapidity, according to the 

 nature of the materials employed, that is to say, according to 

 the specific surface-tensions which are involved. For instance if 

 we mix up a little powdered glass with chloroform, and set a drop 

 of the mixture in water, the glass particles gather neatly round 

 the surface of the drop so quickly that the eye cannot follow the 



* Rhumbler, Das Protoplasma als physikalisches System, Jena, p. 591, 1914; 

 also in Arch. f. Entwickelungsmech. vn, pp. 279-335, 1898. 



