694 ON CONCRETIONS, SPICULES, ETC. [ch. 



organisms biologically remote, and that, in general, mere formal 

 likeness may be a fallacious guide to evolution in time and to 

 relationship by descent and heredity. 



If I have dealt comparatively briefly with the inorganic skeletons 

 of sponges, in spite of the interest of the subject from the physical 

 point of view, it has been owing to several reasons. In the first 

 place, though the general trend of the phenomena is clear, it must 

 be admijited that many points are obscure, and could only be 

 discussed at the cost of a long argument. In the second place, the 

 physical theory is too often (as I have shewn) in conflict with the 

 accounts given by embryologists of the development of the spicules, 

 and with the current biological theories which their descriptions 

 embody; it is beyond our scope to deal with such descriptions 

 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. 



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 

 naturaUsts, but some fifty 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. 

 But apart from the extraordinary multiplicity of forms among the 

 Radiolaria, there are certain features in this multiphcity which 

 arrest our attention. Their distribution 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 geo- 

 graphical habitat; some occur in the neighbourhood of the two 

 poles, some are confined to warm and others to cold currents of 

 the ocean. In time 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 Hving." Lastly, except perhaps in the case of a few 

 large "colonial forms," we seldom if ever find, as is usual in most 

 animals, a local predominance of one particular species. On the 



* 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 \i?ork. 



