790 SUMMARY OF CURRENT RESEARCHES RELATING TO 



By way of proof of this assertion we may cite the interesting 

 discussion which took place * on the sujjposed identity of Nnvicula 

 rhomboides Ehr., Navicula crassinervis Breb.,aud Frustulia saxonica Eab,, 

 a discussion in which Dr. Dallinger, Dr. Wallich, and Messrs. Slack 

 and Ingpeu took part. This controversy arose, less from the point of 

 view of the study of the Diatomacese than in regard to the adoption of 

 their valves as tests for the quality of objectives. In this discussion 

 the illustrious microscopists, Kitton, in England, and Professor H. 

 L. Smith and Colonel Dr. Woodward, in America, added the weight 

 of their authority before the Eoyal Microscopical Society of London, 

 at the meeting of the 6th December, 1876, on the occasion of a 

 paper by Mr. Dallinger. The very great analogy which exists between 

 these three types is evident and undeniable, but from this analogy 

 can it be concluded that there is identity ? Though I may be taxed 

 with temerity, I will take the liberty of replying to this pleiad of 

 illustrious naturalists that this deduction does not seem to me to be 

 just, and that if we are to rely on such arguments in the study of 

 nature we shall soon upset all classification and destroy all notion of 

 species. Being obliged by the extreme smallness of the organisms 

 to content ourselves with reasoning on that which materially and 

 actually appears to our eyes, supplying by inferences from proba- 

 bility, the want of reliable arguments furnished by experience, it 

 seems to me that we must consider the following reasoning sound: 

 When in a sufficiently abundant and pure collection of diatoms 

 several forms resembling each other shall be found, which by 

 numerous specimens of different sizes represent a continuous gra- 

 dation between the two extremes of the series, then only shall we be 

 able reasonably to conclude that these different forms belong to a 

 single species. Whence it results that a deposit of fossil diatoms 

 can never furnish a basis for probable deductions on this matter, 

 since in this case we have to do with a singular mixture of species of 

 different genera and not with an assemblage which can be considered 

 as the components of a single stock or family, or of the same 

 generation. 



Eeturning to the assumed identity of the three types above de- 

 scribed, no mention has been made in the discussion to which it has 

 given rise of any gathering in which these three types have been found 

 united, unless it is in the deposit of Cherryfield, in America, in which 

 only Navicula rJiomhoides and N. crassinervis exist. Also, not being 

 aware up to the present time of any gathering of living diatoms in 

 which the three forms are found united, it seems to me a sufficient 

 argument against admitting for the present their supposed identity. 

 This argument, however, is only negative, and as there is no proof the 

 identity remains possible. But there is a positive argument which, 

 if I am not mistaken, refutes that which considers the three forms 

 other than three distinct and autonomous types. If we examine 

 attentively the three diatoms, it will be found that the transverse 

 striation of Frustulia saxonica of the Typen-platten of Moller has 



* Mon. Micr. Journ., xvii. (1877) pp, 1, 73, and 173. 



