ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 793 



liarity like nodules) remains perfectly constant and uniform, we 

 recognize that all tlie frustules of the collection, notwithstanding the 

 diiferences in size and outline, belong to the same species. 



One of the most favourable circumstances for my views was 

 afforded more than twelve years since, when I was permitted, thanks to 

 the courtesy of M. Alphonse de Candolle) to select from the very rich 

 herbarium of the illustrious botanist of Geneva, some small fragments 

 of Utricularise coming fi-om very different localities, and whicb I 

 found incrusted with diatoms. One of these Utricularise came from 

 Kio de Janeiro, another from Java, and a third from Senegal. All 

 three were equally covered with frustules of Eunotia formica Ehr. In 

 reference to this fact, I may call attention to the singular peculiarity 

 that the same very rare sjpecies of diatom is always found on the 

 same plant, although the latter comes from three very different parts 

 of the globe, a circumstance which may, in my opinion, lead to the 

 conclusion that the Diatomacese are not only epiphytes, but parasites. 

 These three different gatherings all showed the characteristic form 

 of Eunotia formica, with the angular swelling in the centre, the 

 intermediate constrictions, and all the other characters of nodules 

 and pseudo-nodules and moniliform transverse lines, interrupted 

 by an excentric hyaline line, and unequally distributed. Moreover, 

 the three gatherings contained numerous frustules of very different 

 size and outline, but all presenting the same peculiarities of structure ; 

 thus not leaving the least doubt that they all belonged to the same 

 species, Eunotia formica. The longitudinal axis in the largest forms 

 was five times tlie size of the same axis in the smallest forms, and in 

 the latter no indication of the central swelling could be seen, their 

 aspect being rather linear, or showing a slight constriction. I have 

 before me a series of photographs which 1 have made, representing 

 the entire series of these different forms : looking at the distance of 

 the stria3 (especially in the largest and most characteristic specimens), 

 these being very close at the extremities, wider in the centre, and still 

 wider in the two intermediate constrictions; following the gradual 

 variations of outline and size of the frustules, it is impossible not to 

 see that these represent nothing else than different stages of develop- 

 ment, and that in this diatom there is at least a bilateral auxesia, 

 and perhaps also a simultaneous dilatation or stretching of the 

 middle parts between the centre and the extremities. The different 

 forms of this diatom have also been seen by A. Grunow, and figured 

 in the paper which he published on the fresh-water Diatomaceae of the 

 Island of Banka. 



When we consider the uninterrupted series of sizes and outlines, 

 all the structural characters remaining at the same time absolutely 

 identical, we are necessarily led to regard all these frustules, although 

 of different dimensions and outline, as belonging to the same species, 

 and representing simply different degrees of organic development. 

 But in this comparison, the character the constancy of which is the 

 most difficult to establish, is the fineness of the striee which ornament 

 the valves, that is, the number contained in a fraction of a millimetre, 

 so that the invariability of this character in frustules of different 



