794 SUMMARY OF CURRENT RESEARCHES RELATING TO 



sizes belonging to the same species may be verified. It seems to me 

 so much the more necessary to demonstrate this fact in an irrefragable 

 manner, because it is precisely on this point that there is the greatest 

 disagreement in the opinions of those who are regarded as having 

 most authority in the matter, and, moreover, the deductions which 

 may be drawn from it would be of the highest importance to 

 elucidate the biological laws to which the whole family of the 

 Diatomaceae are subservient. Amongst the observers who are the 

 most able and of the most authority, must be first mentioned 

 Dr. Wallich, whom I have the honour to know personally, but whom, 

 entirely from the love of truth, I am obliged to contradict. At the 

 meeting of the Royal Microscopical Society of London, of the 

 3rd January, 1877, he read a paper " On the Eelations between 

 the Development, Reproduction, and Markings of the Diatomaceae," * 

 in which he said that " whilst the total number of the strise on 

 the valves of a diatom may remain nearly uniform in every valve of the 

 same species, the number of striae on the fractional part of a valve ( as 

 a thousandth of an inch) admits of just as much variation as the size 

 of the valve, and proceeds simultaneously with it during division, 

 but not afterwards," This opinion could not be enunciated more ex- 

 plicitly, but is it in accordance with exjierience ? Let us see. 

 From the results of the microphotographic method above described, I 

 am able to demonstrate to Dr. Wallich that his view is not sound, that 

 he has without doubt been misled by a preconceived idea, and that 

 he has not verified the fact by personal exj)erience, relying probably 

 too much on the different figures given for the striae in a particular 

 fraction, by dificrent microscopists who have employed the faulty 

 process of the eye-piece micrometer, f 



Every time that by a happy chance I have been able to discover 

 a mass of diatoms all belonging to a single species, mixed with 

 which were frustules of another genus, I have endeavoured to profit by 

 the occasion to make an attentive examination, and to ascertain the 

 nature of the deviations from the typical form which constitute 

 the idiosyncrasy of the species. On these occasions, after having 

 recognized the types and sizes of the frustules which difier the 

 most, but which are connected by a continuous series of frustules of 

 intermediate size and form, so that no doubt can exist as to the 

 identity of the species to which they belong, I reproduce, by means of 

 microphotography, the image of the smallest and of the largest 

 frustules, always adoj)ting the amplification of 535 diameters. Pro- 

 jecting successively the two images on the wall, I count the striae 

 contained in j^i^j- mm. ; and up to the present time I have never found a 

 single case in which the number of the striae is not constant. In two 

 memoirs which I have published in the ' Proceedings ' of this 

 Society, one under the title of " Nuovi argomenti a provare che le 



* Mon. Micr. Journ., xvii. (1877) p. 61. 



t Dr. Schumann, in ' Die Diatomeeu der Hohen Tatra,' says that the structure 

 of the frustule and closeness of the striae depend on the elevation above sea- 

 level. The higher the habitat the smaller the frustule and the closer the strise. 

 Mon. Micr. Journ., xiii. (1875) p. 265.— Ed. 



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