express the effects of struggle with environment more accurately than do any others, for it is with the hard parts that the animal has met environment, struggled with and resisted it; hence, fossils, so imperfect as evidence of the anatomical structure of the organisms, are the best of evidence of the effect of the interaction between the forces of ancestry, working through the laws of generation tending to repeat the ancestral characters, and the forces of the environment working through the laws of struggle for existence in modifying those characters by adjustment. Kinds of Hard Parts of the Animal Kingdom preserved as Fossils. As we deal, then, with the hard parts only, a few words will be said regarding the kind of hard parts which are found in the several classes of the Animal Kingdom. We glance over the Animal Kingdom and see that there are large groups of animals now living, which, if they were to die and every advantage were offered for their preservation in their natural habitat, would leave no trace of their existence a year after their death. It is important, therefore, to learn at the outset to what extent the paleontological record will be found silent because of impossibility of preservation of the evidence. Protozoa. Among the lowest group of animals, the subkingdom Protozoa, the Gregarinidæ, found mainly within other animals, would be absent because they form no hard parts nor framework which could be preserved. Among the Rhizopoda, differing from the former class in FIG. 10.-Foraminifera. Globigerina bulloides d'Orb. Miocene. (S. and D.) the possession of pseudopoda, and leading a more active and independent life, the orders of Monera and Amaba, as far as known, do not develop any structure which would be likely to escape disintegration and resolution in the ordinary process of fossilization. But the other two orders of the class, Foraminifera, Radiolaria, develop hard skeletons of lime or silica, and great numbers of them are preserved in a fossil state. The Infusoria (a higher class than the others, in the possession of mouth and vibratile or contractile cilia) are not known to exist in a fossil state, though now abundant under proper conditions, and though most probably they lived in like conditions back to earliest geologic time. Figures 10, II. Cœlenterata. Of the Colenterata the classes Spongia and FIG. 12.-Spongia. Astylospongia præmorsa Gf. sp. Silurian. (S. and D.) A, vertical sec tion; B, lateral view; C, silicious skeleton, greatly enlarged. Anthozoa and the Hydroid Zoöphytes (Hydrozoa) are represented. All of the orders of the Anthozoa have families producing some hard parts, "corals," which are preserved in the rocks, but in each order there are some families not developing calcareous skeletons, hence not preserved; and in the Hydrozoa (class) several orders and a few whole subclasses (as the Lucernarida, Siphonophora, etc.) are of such a nature as to be wanting in any geologic record, and therefore in so far the history of the Coelenterata is necessarily imperfect. However, Corals are among the most abundant fossils, and Graptolites (related probably to the Hydroid Polyps, or Sertularida) are also abundant in a few zones in the Paleozoic rocks. Figures 12, 13, 14. FIG. 13. FIG. 14. FIG. 13.-Graptolite. Diplograptus palmeus Barr. Silurian. (S. and D.) FIG. 14.-Coral. Parasmilia centralis Mant. sp. Cretaceous A, corallite, longitudinally sectioned; B, the same seen from above; s and 1-5 septa, c = columella. Echinodermata were represented in fossil form, developing some hard parts in each order, viz.: Crinoidea, Blastoidea, Cystidea, Ophiuroidea, Asteroidea, Echinoidea, and even the Holothurioidea probably recognized in the spiculæ. The Solecida (parasitic worms, whether grouped with the Echinodermata, or with annelids under Vermes) are all soft, and do not come within the province of the paleontologist. Figures 15-19. Vermes. Among the Vermes (the leeches, earthworms, and sea-worms) there are some which produce earthy cases of mud, others have left their tracks where they bored through the tenacious mud; also teeth have been found, supposed to belong to this group. (See Serpula, Spirorbis, etc.) Still, these are rare fossils, and probably represent but very imperfectly the worms living in ancient seas. Figure 20. Arthropoda. Of the Arthropoda, including all those animals composed of definite segments arranged longitudinally, one behind the other, and the locomotor appendages of which are jointed or articulated to the body, we have four great classes: Crustacea, Arachnida, Myriapoda, Insecta. All of these produce a more or less enduring, horny or calcareous crust or case, within which the soft parts are contained, making the FIG. 15-Echinodermata, Crinoid. Taxocrinus multibrachiatus Ly. and Cass. Carboniferous, Above: calyx with stem. Below: the plates of the calyx dissected. st = stem, br= free arms, air = anal interradial plates; h = right posterior, th= left posterior, vr anterior right, != anterior left, zu anterior medial radial plates; irh right posterior, ilh left posterior, irv = right anterior, il left anterior interradial plates; if, infrabasalia; pb = parabasalia = radialia; dI, dII distichalia, first and second rank; br brachialia; a1-45 = anal plates; irir, larger interradial plates; t = smaller interradial plates; ss, plane of symmetry. (After S. and D) Devonian. b= basalia; r = FIG. 16. Blastoid. Pentatremitidea Eifeliensis F. Rö. sp. radialia; ambulacra. (After S. and D.) FIG. 17.-Cystoid. Caryocrinus ornatus Say. Silurian. A, calyx with stem s; br arms; I, II, III, first, second, and third zones of plates of the dorsal capsule; porous plates; i = place of attachment of arms; a = anal opening B, view of the ventral dome, c = central summit plates, i and a as above. C, inner surface of a plate of the second row, showing the pores of the Hydrospires (p) and their connecting canal (c). (After S. and D.) FIG. 18. -Ophiuroid, A-H. Ophioceramis ferruginea Böhm. Jurassic. A, a complete specimen, from under side; br arms. B = the disk from the inner side; bl bursal shields; ambulacral pores. C, mouth-skeleton from below; m = mouth-angle; p = papilla; me angle-plates; s= side shields; m = oral plates; b= second ventral plate. D, disk from above; dorsal shield. E, a part of an arm from below; b = ventral shields; s = lateral shields; ambulacral pores. F, a part of an arm from above; and s as above; st = spines G, the same. lateral view. H, cross-section of an arm. 1, Geocoma planata Qu. sp.; bs bursal slit; b = bursal shield. (Steinmann and Doederlein.) = FIG. 20. opening; am paired ambulacral plates with two double rows of pores and small spines; ia single row of interambulacral plates. B, summit region with the anal opening (a). C, under side, with the mouth opening. FIG. 20.-Vermes, Annelida Serpula. A, S. (Spirorbis) omphalodes Gf. Devonian. B, C, S. (Galeolaria) socialis Gf. Jurassic. C, cross-section of the tubes. D, Serpula gordialis Schl. Cretaceous. E, S. (Rotularia) spirulaa Lam. Tertiary. Crustacea, it will be observed that the animals belonging to these classes live mainly on land. and in the air, and when we bear in mind that fossilization is a process usually requiring water for the preparation of the matrix (sand, mud, gravel, etc.), and for the covering of the body with the material when prepared, it is evident that all land and aërial animals, although possessing parts capable of fossilization, and living in abundance, run very small chance of being found in the deposits made. under water, in which fossils are mainly preserved. Hence Crustacea, being water animals, are preserved as fossils in considerable numbers, while the other classes of Arthropoda, that is, insects, spiders, and rfi T FIG. 21.-Arthropod, Crustacean. Trilobite, = Myriapods, although occasionally found, are rare, and prob |