IX] OF THE RADIOLARIAN SKELETON 457 



in detail. Lastly, we find ourselves able to illustrate the same 

 physical principles with greater clearness and greater certitude in 

 another group of animals, namely the Radiolaria. In our descrip- 

 tion of the skeletons occurring within this group we shall by no 

 means abandon the preliminary classification of microscopic 

 skeletons which we have laid down ; but we shall have occasion 

 to blend with it the consideration of certain other more or less 

 correlated phenomena. 



The group of microscopic organisms known as the Radiolaria 

 is extraordinarily rich in diverse forms, or "species." I do not 

 know how many of such species have been described and defined 

 by naturalists, but some thirty years ago the number was said 

 to be over four thousand, arranged in more than seven hundred 

 genera*. Of late years there has been a tendency to reduce the 

 number, it being found that some of the earlier species and even 

 genera are but growth-stages of one and the same form, sometimes 

 mere fragments or "fission-products" common to several species, 

 or sometimes forms so similar and so interconnected by inter- 

 mediate forms that the naturalist denominates them not "species" 

 but "varieties." It has to be admitted, in short, that the con- 

 ception of species among the Radiolaria has not hitherto been, 

 and is not yet, on the same footing as that among most other 

 groups of animals. But apart from the extraordinary multiplicity 

 of forms among the Radiolaria, there are certain other features 

 in this multiplicity which arrest our attention. For instance, 

 the distribution of species in space is curious and vague ; many 

 species are found all over the world, or at least every here and 

 there, with no evidence of specific limitations of geographical 

 habitat ; others occur in the neighbourhood of the two poles ; 

 some are confined to warm and others to cold currents of the 

 ocean. In time also their distribution is not less vague : so much 

 so that it has been asserted of them that "from the Cambrian 

 age downwards, the families and even genera appear identical 

 with those now living." Lastly, except perhaps in the case of 

 a few large "colonial forms," we seldom if ever find, as is usual 



* Haeckel, in his Challenger Monograph, p. clxxxviii (1887) estimated the 

 number of known forms at 4314 species, included in 739 genera. Of these, 3508 

 .species were described for the first time in that work. 



