(§§ 153-200.)

153. Sources of Phylogenetic Knowledge.—For the purpose of constructing a hypothetical genealogical tree of the Radiolaria, as of all other organisms, three sources of information are open to us, viz., palæontology, comparative ontogeny, and comparative anatomy. In the present case, however, these three sources are of very different value; the first two are at present only very inadequately known and have only been partially investigated, hence they can only be utilised to a very slight extent. The comparative anatomy of the Radiolaria, on the other hand, is so completely known, and affords such certain glimpses into the morphological relations of the related groups, that by its aid we are in a position at all events to lay down the general features of their phylogeny with some probability, and to lay the foundation of a natural system.

154. Natural and Artificial Systems.—Although in the classification of the Radiolaria, as in the case of all other organisms, the natural system must be regarded as the goal of systematic classification, our phylogenetic knowledge of the Radiolaria is too fragmentary and inadequate to admit of the systematic arrangement here adopted being regarded as a thoroughly consistent natural system, that is, as representing the true genealogical tree of the class. Owing, however, to the extraordinary variety of form of the Radiolaria, and the complicated relationships of the larger and smaller groups, a synoptical grouping of the different categories and the erection of a complete, even if to some extent artificial, system, becomes a logical necessity. Under these circumstances, and regard being had to both these conditions, the following systematic treatment of the Radiolaria will appear as a compromise between the natural and artificial systems, like all other zoological and botanical classificatory attempts. On the one hand, the attempt is made to arrange the larger and smaller groups as nearly as possible according to their phylogenetic relationships, whilst, on the other hand, the practice of circumscribing each by a definition as clear and logical as possible has been carried out. Since these two efforts naturally often come into contact, the insufficiency of many parts of the arrangement is obvious, hence its hypothetical and provisional character is emphatically stated.

155. Systematic Categories.—The categories or different orders of divisions have in the Radiolaria, as in all other organisms, no absolute significance, but only a relative value. In itself it is quite unimportant whether the whole group be regarded, as at first, as a family (Ehrenberg, 1847), or as an order (J. Müller, 1858), or as a class (Haeckel, 1881). These different views are regulated, on the one hand, by the known extent of the group and by the amount of our acquaintance with it, and on the other, by comparison with related groups and by reference to their conventional disposition. When, therefore, the whole class, Radiolaria, is here divided into two subclasses, four legions, eight orders, eighty-five families, &c., these artificial categories are drawn up only in the conviction that by this means the easiest survey and most thorough insight into the system as a whole may be attained; this latter will indeed approach as far as possible the ideal of a natural system, but must on numerous practical grounds always remain more or less artificial. Since it is to be expected that with the progress of our systematic knowledge the rank of the various categories will rise, it is possible that in the future the arrangement of the group may be somewhat as follows:—Phylum, RADIOLARIA; Four Classes, Spumellaria, Nassellaria, Acantharia, Phæodaria; Eight Legions (Nos. I.-VIII. in the following Table); Twenty Orders (Nos. 1-20 in the Table), &c.

Four Legions.Eight Sublegions.Twenty Orders.Typical Families.
I. Legion (or Subclass)
Spumellaria
(Peripylea)
[Porulosa peripylea.]		I. Collodaria
(Spumellaria palliata)		1. Colloidea,1a. Thalassicollida.
1b. Collozoida.
2. Beloidea,2a. Thalassosphærida.
2b. Sphærozoida.
II. Sphærellaria
(Spumellaria loricata)		3. Sphæroidea,3a. Ethmosphærida.
3b. Collosphærida.
4. Prunoidea,4a. Ellipsida.
4b. Zygartida.
5. Discoidea,5a. Phacodiscida.
5b. Porodiscida.
6. Larcoidea,6a. Larnacida.
6b. Pylonida.
II. Legion (or Subclass)
Acantharia
(Cannopylea).
[Osculosa cannopylea.]		III. Acanthometra
(Acantharia palliata)		7. Actinelida,7a. Astrolophida.
7b. Litholophida.
7c. Chiastolida.
8. Acanthonida,8a. Astrolonchida.
8b. Quadrilonchida.
8c. Amphilonchida.
IV. Acanthophracta
(Acantharia loricata)		9. Sphærophracta,9a. Sphærocapsida.
9b. Dorataspida.
9c. Phractopeltida.
10. Prunophracta,10a. Belonaspida.
10b. Hexalaspida.
10c. Diploconida.

III. Legion (or Subclass)
Nassellaria
(Monopylea)
[Osculosa monopylea.]

V. Plectellaria
(Nassellaria palliata)		11. Nassoidea,11. Nassellida.
12. Plectoidea,12a. Plagonida.
12b. Plectanida.
13. Stephoidea,13a. Stephanida.
13b. Tympanida.
VI. Cyrtellaria
(Nassellaria loricata)		14. Spyroidea,14a. Zygospyrida.
14b. Androspyrida.
15. Botryodea,15a. Cannobotryida.
15b. Lithobotryida.
15c. Pylobotryida.
16. Cyrtoidea,16a. Monocyrtida.
16b. Dicyrtida.
16c. Tricyrtida.
16d. Stichocyrtida.
IV. Legion (or Subclass)
Phæodaria
(Actipylea)
[Porulosa actipylea.]		VII. Phæocystina
(Phæodaria palliata)		17. Phæocystina,17a. Phæodinida.
17b. Cannorrhaphida.
17c. Aulacanthida.
18. Phæosphæria,18a. Orosphærida.
18b. Aulosphærida.
18c. Cannosphærida.
VIII. Phæocoscina
(Phæodaria loricata)		19. Phæogromia,19a. Challengerida.
19b. Castanellida.
19c. Circoporida.
20. Phæoconchia,20a. Concharida.
20b. Cœlodendrida.
20c. Cœlographida.
Four Legions.
Eight Sublegions.
Twenty Orders.
Typical Families.

I. Legion (or Subclass) (Peripylea)

[Porulosa peripylea.]

I. Collodaria (Spumellaria palliata)
1. Colloidea,
1a. Thalassicollida.
1b. Collozoida.
2. Beloidea,
2a. Thalassosphærida.
2b. Sphærozoida.
II. Sphærellaria (Spumellaria loricata)
3. Sphæroidea,
3a. Ethmosphærida.
3b. Collosphærida.
4. Prunoidea,
4a. Ellipsida.
4b. Zygartida.
5. Discoidea,
5a. Phacodiscida.
5b. Porodiscida.
6. Larcoidea,
6a. Larnacida.
6b. Pylonida.

II. Legion (or Subclass) Acantharia (Cannopylea).

[Osculosa cannopylea.]

III. Acanthometra (Acantharia palliata)
7. Actinelida,
7a. Astrolophida.
7b. Litholophida.
7c. Chiastolida.
8. Acanthonida,
8a. Astrolonchida.
8b. Quadrilonchida.
8c. Amphilonchida.
IV. Acanthophracta (Acantharia loricata)
9. Sphærophracta,
9a. Sphærocapsida.
9b. Dorataspida.
9c. Phractopeltida.
10. Prunophracta,
10a. Belonaspida.
10b. Hexalaspida.
10c. Diploconida.

III. Legion (or Subclass) Nassellaria (Monopylea)

[Osculosa monopylea.]

V. Plectellaria (Nassellaria palliata)
11. Nassoidea,
11. Nassellida.
12. Plectoidea,
12a. Plagonida.
12b. Plectanida.
13. Stephoidea,
13a. Stephanida.
13b. Tympanida.
VI. Cyrtellaria (Nassellaria loricata)
14. Spyroidea,
14a. Zygospyrida.
14b. Androspyrida.
15. Botryodea,
15a. Cannobotryida.
15b. Lithobotryida.
15c. Pylobotryida.
16. Cyrtoidea,
16a. Monocyrtida.
16b. Dicyrtida.
16c. Tricyrtida.
16d. Stichocyrtida.

IV. Legion (or Subclass) Phæodaria (Actipylea)

[Porulosa actipylea.]

VII. Phæocystina (Phæodaria palliata)
17. Phæocystina,
17a. Phæodinida.
17b. Cannorrhaphida.
17c. Aulacanthida.
18. Phæosphæria,
18a. Orosphærida.
18b. Aulosphærida.
18c. Cannosphærida.
VIII. Phæocoscina (Phæodaria loricata)
19. Phæogromia,
19a. Challengerida.
19b. Castanellida.
19c. Circoporida.
20. Phæoconchia,
20a. Concharida.
20b. Cœlodendrida.
20c. Cœlographida.

156. Formation of Species.—The totality of similar forms, which we unite in one species, and which in the earlier dogmatic systems was regarded as a category of absolute value, possesses only a relative value like all other systematic categories (§ [155]). According to the individual views of the systematist and the general survey which he has attained of the smaller and larger systematic groups, the conception of a species adopted in his practical work will be wider or narrower. In the present systematic arrangement a medium extent has been adopted. It is shown that in the Radiolaria, as in all other extensive groups of organisms, the constancy of the species is very variable in the different groups. Many families of Radiolaria are very rich in "bad species," i.e., very variable forms, in which the process of the formation of species is seen in progress; such, for example, are—among the Spumellaria, the Sphærozoida, Stylosphærida, Phacodiscida and Pylonida; among the Acantharia, the Amphilonchida and Phractopeltida; among the Nassellaria, the Stephoidea and Botryodea; and among the Phæodaria, the Aulacanthida, Sagosphærida, Castanellida and Concharida. On the other hand, in some families numerous "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 Astrosphærida, Cyphinida, Porodiscida and Tholonida; among the Acantharia the Quadrilonchida and Dorataspida; among the Nassellaria, the Spyroidea and Cyrtoidea; among the Phæodaria, the Challengerida, Medusettida, Circoporida and Cœlographida. 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 bonæ" were "malæ." There may be observed in the manifold skeletal forms of the Radiolaria, on the one hand, the utmost accuracy of configuration, 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. Palæontological Development.—The palæontology 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 Palæozoic Radiolaria (from the coal measures) only slight traces are known. Moreover, the fossil Radiolaria hitherto known have been found only in very circumscribed and widely separated localities. The majority of all the species belong to the small island of Barbados. Although our palæontological acquaintance 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 Radiolaria, the Acantharia (on account of the solubility of their astroid acanthin skeletons) have entirely vanished and have never been found fossil. Of the Phæodaria, whose silicate skeleton is not as a rule capable of fossilisation, only one section (Dictyochida) of a single family (Cannorrhaphida) has been observed fossil. Hence the fossil remains of the Radiolaria belong almost exclusively to the two legions, Spumellaria 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 Larcoidea (among the Spumellaria) and of the Stephoidea (among the Nassellaria) with a few isolated exceptions, no fossils are known. The great majority of fossil Radiolaria belong to the two Nassellarian orders Cyrtoidea and Spyroidea (two relatively very highly developed groups); next to these follow the orders Discoidea and Sphæroidea among the Spumellaria. From these palæontological facts it is obvious that our present very incomplete acquaintance with the fossil Radiolaria is quite insufficient to warrant us in drawing any conclusions from it regarding the phylogenetic development or palæontological succession of the individual groups.

158. Origin of the Four Legions.—The agreement of all Radiolaria in those constant and essential characters of the unicellular body, which distinguish them from all other Protista (especially the differentiation of the malacoma into a central capsule and extracapsulum), justifies the conclusion that all members of this class have been developed from a common undifferentiated stem-form. Only the simplest form of the Spumellaria, a skeletonless spherical cell with concentric spherical nucleus and calymma, can be regarded as such. The simplest form of the Thalassicollida which is now extant (Actissa, Procyttarium, p. [12]), corresponds so exactly to the morphological idea of that hypothetical stem-form that it may unhesitatingly be regarded in a natural system as the common point of origin of the whole class. On the other hand, Actissa is so closely related to the simple Heliozoa (Actinophrys, Actinosphærium, Heterophrys, Sphærastrum, &c.) that its origin from this group of Rhizopoda is exceedingly probable. The three legions Acantharia, Nassellaria, and Phæodaria are to be regarded as three main diverging branches of the genealogical tree, which have been developed in different directions and are only connected by their simplest stem-forms (Actinelius, Nassella, Phæodina) with the stem-form of the Spumellaria, the primordial Actissa.