civ THE VOYAGE OF H.M.S, CHALLENGER. 



other hand, in some families mimerous " good species " may be distinguished, since the 

 intermediate connecting forms are no longer present and the forms have become relatively 

 constant. As instances of such families may be mentioned, among the Spumellaria, 

 the Astrosphserida, Cyphinida, Porodiscida and Tholonida ; among the Acantharia 

 the Quadrilonchida and Dorataspida ; among the Nassellaria, the Spyroidea and 

 Cyrtoidea; among the Ph^odaria, the Challengerida, Medusettida, Circoporida 

 and Coelographida. The more carefully the different groups are studied, the more 

 numerous the individuals of each species under comparison, the greater becomes the 

 number of " bad" species among the Radiolaria, and the smaller the number of good ones. 

 Originally, no doubt, all " species bonae " were " malse." There may be observed in the 

 manifold skeletal forms of the Radiolaria, on the one hand, the utmost accuracy of con- 

 figuration, and on the other, the greatest variability, and hence a careful comparative study 

 of them leads to a firm conviction of the gradual " Transformation of Species," and of the 

 truth of the " Theory of Descent." 



157. Palceontological Development. — The palaeontology of the Radiolaria already 

 offers very considerable material for study ; but in consequence of its incompleteness this 

 is of little value for the study of the phylogeny of the class. By far the larger portion 

 of the fossil Radiolaria belong to the Tertiary period ; only quite recently have numerous 

 well-preserved fossil Radiolaria been described from the Mesozoic period, and especially 

 from the Jura. Of Palaeozoic Radiolaria (from the coal measures) only slight traces are 

 known. Moreover, the fossil Radiolaria hitherto known have been found only in very cir- 

 cumscribed and widely separated localities. The majority of all the species belong to the 

 small island of Barbados. Although our palseontological accjuaintance with the Radiolaria 

 must necessarily be incomplete for this reason, it is still more so since at least thirty 

 out of the eighty-five families (that is more than a third) could not possibly leave any fossil 

 remains, either because they possess no skeleton, or because of its chemical composition. 



Of the four legions of the Eadiolaria, the Acakthakia (on account of the solubility of their 

 astroid acanthiu skeletons) have entirely vanished and have never been found fossil. Of the 

 PHi5:oDAEiA, whose silicate skeleton is not as a rule capable of fossihsation, only one section (Dictyo- 

 chida) of a single family (Cannorrhaphida) has been observed fossil. Hence the fossil remains of 

 the Eadiolaria belong almost exclusively to the two legions, Spumellaeia and Nassellaria, which 

 were formerly united under the term " Polycystina." Among these, however, the skeletonless 

 Thalassicollida, Collozoida, and Nassellida could leave no traces. Hence there only remain fifty- 

 five families of which we might expect to find fossil siliceous skeletons. Even of these, however, 

 scarcely the half are certainly known in the fossil condition, whilst of the remainder nothing certain 

 is known ; for example, of the large order L a r c o i d e a (among the Spumellaria) and of the 

 Stephoidea (among the Nassellaria) with a few isolated exceptions, no fossils are known. 

 The great majority of fossil Eadiolaria belong to the two Nassellaeian orders Cyrtoidea and 

 Spyroidea (two relatively very highly developed groups); next to these follow the orders 



