IX] OF AGGLUTINATED SKELETONS 705 



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 httle grains of quartz. 

 Even this phenomenon may be accounted for on surface-tension 

 principles, if we may 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; we are by 

 no means obhged to attribute even this phenomenon to a mani- 

 festation of volition, or taste, or aesthetic skiU, on the part of the 

 microscopic organism. Nor, when certain Radiolaria tend more 

 than others to attract into their own substance diatoms and such- 

 hke foreign bodies, is it scientifically correct to speak, as some 

 text-books do, of species "in which diatom-selection has become 

 a regular habits To do so is an exaggerated misuse of anthropo- 

 morphic 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 important consequences in 

 biology. For the shell to assume the sohd and permanent character 

 which it acquires in Difflugia, we have only to make the further 

 assumption that small quantities of a cementing substance are 

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

 capillary attraction through all the interstices . of the little quartz 

 grains, and ends by binding them 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. 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 operation. If we do the same with oil 

 and fine sand, dropped into 70 per cent, alcohol, a still more 



* Rhumbler, Beitrage z. Kenntniss d. Rhizopoden, i-v, Z. f. w. Z. 1891-o; Das 

 Protoplasma als physikalisches System, Jena, p. 591, 1914; also in Arch. J. Ent- 

 wickelungsmech. vii, pp. 279-335, 1898; Biol. Centralbl. xvm, 1898; etc. 



