OF HETEROGBNESIS 203 



(a) The Diatoms have, like the Algje, obtained entry to the sub- 

 epidermal spaces through the stomata. 



(6) The Diatoms have been produced in situ by a transformation 

 of the fission products of the Algas. 



The first of these possibilities it will be convenient to speak of 

 as the Infection Hypothesis, and the second as the Transformation 

 Hypothesis. 



{a) Infection Hypothesis. The difficulty in accounting for the 

 facts seem to me to be extreme in accordance with this supposition, 

 especially if we bear in mind what is authoritatively known con- 

 cerning Diatoms. The important points are these : — 



1. No vwtile spores are known ; and previous to 1896 there was 

 no certain knowledge concerning the existence of spores of any 

 kind in Diatoms. The important discoveiy by Geo. Murray of 

 undoubted spores or germs, originating by a process of rejuvenes- 

 cence, in species belonging to three marine genera, constitutes all 

 that is certainly known at present on this subject.' 



2. It is commonly stated by writers that individual Diatoms do 

 not increase in size ^ ; increase in bulk of Diatoms being only 

 brought about as a result of conjugation, which is admitted to be a 

 comparatively rare process. 



3. Previous to the above-mentioned discovery by Geo. Murray, 

 Diatoms were said to be formed only (i) by a process of conjuga- 

 tion, or (2) by fission — the latter being the common process, and one 

 that is generally said to involve a very slight diminution in size of 

 the products.3 



Such facts, concerning Diatoms in general, must be borne in mind 

 in conjunction with these others, more especially bearing upon the 

 question now under consideration. 



4. The sub-stomatal spaces that either are, or have been, tenanted 

 by Chlorochytrium probably constitute much less than ten per 

 cent, of those existing on most leaves of the Duckweed, yet no 

 Diatoms are ever to be seen in the other ninety per cent, of the 

 sub-stomatal spaces. 



5. The purposeless to and fro movements of some Diatoms 

 when free in a fluid, and their absence of movement when lying 

 on the surface of a leaf, seem quite incompatible with the notion 



' " Proceed, of Roy. See. of Edin.," 1896-7, p. 207. 

 = WoUe, " Diatomaceae of North America," 1890, p. 11. 



3 Smith's "British Diatomaceae," vol. i., 1853, p. xxiv., and vol. ii., 1856, p. vii.; 

 and Pritchard, " History of the Infusoria," fourth edition, 1861, pp. 58, 61-63. 



