1760 THE VOYAGE OF H.M.S. CHALLENGER. 



NOTE ON THE DIMENSIONS AND MEASUREMENTS OF THE DESCRIBED 



SPECIES OF RADIOLARIA. 



All the dimensions of the species of Radiolaria described in the present work are 

 given in milhmetres. In the majority of the species the dimensions of only a single 

 observed specimen have been measured by the micrometer, and usually only the most 

 important proportions have been recorded. But since the outlines of nearly all the 

 species figured (with very few exceptions) have been drawn by the camera lucida, 

 and therefore usually are almost perfectly exact, and since the excellent artist, Mr. 

 Adolph Giltsch, has executed the lithograj)hic plates with the greatest accuracy, having 

 examined the objects themselves under the microscope, it is very easy to determine 

 the dimensions of all the separate parts by comparative measurement. In very many 

 of the species described (perhaps nine hundred or a thousand) several specimens of 

 each species (usually three or four) have been measured comparatively, and the 

 dimensions recorded are taken as averages. A very important contribution to the 

 general conception of the proportions, and especially to the important question of the 

 constancy of the dimensions, has been given by my honoured friend, Dr. Reinhold 

 Teuscher of Jena. This excellent observer, to whom I am indebted for much and 

 important co-operation in my Radiolarian work, has instituted at my request a long 

 series of measurements, with the view of comparing the dimensions' (of the entire 

 skeleton as well as of its individual parts) in numerous (usually twenty or thirty) 

 specimens of one and the same species. About three hundi'ed species of very difi"erent 

 groups (mainly Sphseroidea, Discoidea, Spyroidea, and Cyrtoidea) have 

 been measured in this manner, and the general survey of the results obtained (about 

 eight thousand measurements were recorded) has enabled me to form a good opinion of 

 the constancy and variability of the dimensions in the individual species. The general 

 result is, that they are not absolutely constant in any given case, but that each 

 species (of which many specimens have been carefully compared) exhibits a certain 

 degree of variability in all its proportions. The general meaning of " species," 

 therefore, is in the unicellular Radiolaria the same as in all other organisms, and 

 its development follows the same laws as are so accurately explained by Charles 

 Darwin in his Origin of Species. 



