theories which their descriptions embody; it is beyond our scope to deal with such descriptions {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 description of the skeletons occurring within this group we shall by no means abandon the preliminary clasôÙsiôÙfiôÙcaôÙtion 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 ãspecies.ã 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 genera63. 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 ãfission-productsã common to several species, or sometimes forms so similar and so interconnected by intermediate forms that the naturalist denominates them not ãspeciesã but ãvarieties.ã It has to be admitted, in short, that the conception 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 ãfrom the Cambrian age downwards, the families and even genera appear identical with those now living.ã Lastly, except perhaps in the case of a few large ãcolonial forms,ã we seldom if ever find, as is usual {458} in most animals, a local predominance of one particular species. On the contrary, in a little pinch of deep-sea mud or of some fossil ãRadiolarian earth,ã we shall probably find scores, and it may be even hundreds, of different forms.