ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 105 



(3) The main proof of the first hypothesis consists in this, that for 

 the motion of diatoms their contact with some fixed body is indis- 

 pensable. This can only claim to be indisputable on its being 

 ascertained that a floating diatom does not move at all. Up to the 

 present, however, we know of no method for the observation of dia- 

 toms under such conditions, and consequently this suggestion is for 

 the time incapable of being supported. We may suppose that the 

 movable alga clings to the bottom, because (1) as a heavier body it at 

 once sinks, and (2) that having got there, it adheres to the substratum 

 by means of some glutinous substance (as a mucous external cell- 

 membrane). This adherence is anyhow not so firm that it renders it 

 impossible for the osmotic forces to move the cell. Therefore, the 

 facts described might be explained simply and naturally, without the 

 assumption of any invisible protoplasm. And the probability of this 

 explanation is increased by the fact that amongst plants a number of 

 analogous phenomena — that is, examples of adherence of the cells by 

 reason of the local or general mucous condition of their walls — is 

 known to exist. That the Diatomaceee form mucus is borne out by 

 the observations of Borscow : — " The living cells of many species of 

 Coccoiieis, Navicula limosa, and Frustulia appear under very exact 

 focussing to be bordered by a substance as transparent as glass, and 

 having a sharp outline. The observations were made with Hartnack's 

 best objectives, and the possibility of aberration phenomena was not 

 lost sight of."* 



A similar, and, in addition, temporary adherence is seen with some 

 free-living animals, as Eotifera and Infusoria, e. g. with Ervilia mono- 

 styla f and Tintinnus inqmlmus,'^. in which the phenomena depend upon 

 the bottom of the test of the animal becoming mucous. 



The same mucous condition may very well explain the adherence 

 of foreign bodies to Diatomaceee. The adherence casually observed 

 is by no means restricted to the suture, but takes place over the whole 

 surface of the alga.§ In the same way, on this view of the matter, the 

 trailing of sand-grains is easily understood ; fine transparent threads 

 of mucus which proceed from the external mucous layer of the cell, 

 explain this connection of the grains much more simply than any 

 protoplasmic threads or pseudopodia. 



When we weigh what has been said, and compare the proofs which 

 the two parties adduce, it is found that almost all the (indirect) proofs 

 adduced in favour of the protoplasm theory are met successfully by 

 the opponents. We cannot therefore regard the second, or osmotic, 



quoted that some Diatomacese are specifically heavier than water, therefore sink 

 to the bottom ; others, on the contrary, are lighter, and consequently swim. The 

 physico-chemical processes of which the life of the cell is composed, since they 

 alter somewhat the specific gravity of the alga, can by reducing it impart an 

 impulse upwards, as the result of which one end of the alga may be raised from 

 the ground, and thus its horizontal position is changed into an oblique one. 



* Loc. cit., p. 41. 



t Dujardin, ' Infusorres,' p. 455. 



X 0. Mereschkowsky, ' Studien iiber Protozoen des nordlichen Kusslands,' 

 1878, p. 20. 



§ Borscow, loc. cit., p. 42. 



