Volume II Part 23 (1/2)

ii. p. 140. The egg of the Olynthus (Fig. 9), which represents the common ancestral form of all Calcareous Sponges, is a simple cell (Fig.

1). From this there arises, by repeated division (Fig. 2), a globular, mulberry-like heap of numerous equi-formal cells (Morula, Fig. 3; vol.

ii. p. 125.) As the result of the change of these cells into an outer series of clear ciliated cells (Exoderm) and an inner series of dark, non-ciliated cells (Entoderm), the ciliated larva, or Planula, makes its appearance. This is oval in shape, and forms a cavity in its centre (gastric cavity, or primitive stomach, Fig. 6 _g_, with an opening (mouth-opening, or primitive mouth, Fig. 6 _o_); the wall of the gastric cavity consists of two layers of cells, or germ-layers, the outer ciliated Exoderm (_e_) and the inner non-ciliated Entoderm (_i_). Thus arises the exceedingly important stomach-larva, or Gastrula, which reappears in the most different tribes of animals as a common larval form (Fig. 5, seen from the surface; Fig. 6, in long section. Compare, vol. ii. pp. 126 and 281). After the Gastrula has swum about for some time in the sea, it fastens itself securely to the sea-bottom, loses its outer vibratile processes, or cilia, and changes into the Ascula (Fig.

7, seen from the surface; Fig. 8, in long section; letters as in Fig.

6). This Ascula is the recapitulative form, according to the biogenetic fundamental law, the common ancestor of all Zoophytes, namely, the Protascus (vol. ii. pp. 129, 133). By the development of pores in the wall of the stomach and of three-rayed calcareous spicules, the Ascula changes into the Olynthus (Fig. 9.) In Fig. 9 a piece is cut out from the stomach-wall of the Olynthus in order to show the inside of the stomachal cavity, and the eggs which are forming on the surface (_g_).

From the Olynthus the most various forms of Calcareous Sponges can develop. One of the most remarkable is the Ascometra (Fig. 10), a stock or colony from which different species, and in fact different generic forms, grow (on the left Olynthus, in the middle Nardorus, on the right Soleniscus, etc., etc.). Further details as to these most interesting forms, and their high importance for the Theory of Descent, may be found in my ”Monograph of the Calcareous Sponges” (1872), especially in the first volume. (Compare vol. ii. pp. 160, 167).

PLATE I. (_Between pages 184 and 185, Vol. I._)

_History of the Life of the most Simple Organism_, a Moneron (Protomyxa aurantiaca). Compare vol. i. p. 184, and vol. ii. p. 53. The plate is a smaller copy of the drawing in my ”Monographie der Moneren” (Biologische Studien, 1 Heft, 1870; Taf. 1), of the developmental history of the Protomyxa aurantiaca; I have there also given a detailed description of this remarkable Moneron (pp. 11-30). I discovered this most simple organism in January, 1867, during a stay in Lanzarote, one of the Canary Islands; and moreover I found it either adhering to, or creeping about on the white calcareous sh.e.l.ls of a small Cephalopod (vol. ii. p. 162), the Spirula Peronii, which float there in ma.s.ses on the surface of the ocean, or are thrown up on the sh.o.r.e. The Protomyxa aurantiaca is distinguished from the other Monera by the beautiful and bright orange-red colour of its perfectly simple body, which consists merely of primaeval slime, or protoplasm. The fully developed Moneron is represented in Figs. 11 and 12, very much enlarged. When it is hungry (Fig. 11), there radiate from the surface of the globular corpuscule of plasm, quant.i.ties of tree-shaped, branching and mobile threads (pseudo-feet, or pseudopodia), which do not become retiformly connected. When, however, the Moneron eats (Fig. 12), the mucous threads become variously connected, form net-works and enclose the extraneous corpuscule which serves as food, which the threads afterwards draw into the interior of the Protomyxa. Thus in Fig. 12 (above on the right), a silicious and ciliated Whip-swimmer (Peridinium, vol. ii. pp. 51, 57), has just been caught by the extended mucous filaments, and has been drawn into the interior of the mucous globule, in which there already are several half digested silicious infusoria (Tintinoida), and Diatomeae (Isthmia). Now, when the Protomyxa has eaten and grown sufficiently, it draws in all its mucous filaments (Fig. 15), and contracts into the form of a globule (Fig. 16 and Fig. 1). In this state of repose the globule secretes a simple gelatinous covering (Fig. 2), and after a time subdivides into a large number of small mucous globules (Fig. 3). These soon commence to move, become pear-shaped (Fig. 4), break through the common covering (Fig. 5), and then swim about freely in the ocean by means of a delicate whip-shaped process, like the Flagellata (vol. ii.

p. 57, Fig. 11). When they meet a Spirula sh.e.l.l, or any other suitable object, they adhere to it, draw in their whip, and creep slowly about on it by means of form-changing processes (Figs. 6, 7, 8), like Protambae (vol. i. p. 186, vol. ii. p. 52). These small mucous corpuscules take food (Figs. 9, 10), and attain their full grown form (Figs. 11, 12), either by simple growth or by several of them fusing to form a larger protoplasmic ma.s.s (Plasmodium, Figs. 13, 14).

PLATES II. AND III. (_Between pages 294 and 295, Vol. I._)

_Germs or Embryos of four different Vertebrate Animals_, namely, Tortoise (_A_ and _E_), Hen (_B_ and _F_), Dog (_C_ and _G_), and Man (_D_ and _H_). Figs. _A_, _D_, an early stage of development; Figs. _E_, _H_, a later stage. All the eight embryos are represented as seen from the right side, the curved back turned to the left. Figs. _A_ and _B_ are seven times enlarged, Figs. _C_ and _D_ five times, Figs. _E_ and _H_ four times. Plate II. exhibits the very close blood relations.h.i.+p between birds and reptiles; Plate III. that between man and the other mammals.

PLATE IV. (_Between pages 34 and 35, Vol. II._)

_The Hand, or Fore Foot, of nine different Mammals._ This plate is intended to show the importance of Comparative Anatomy to Phylogeny, in as much as it proves how the internal skeleton of the limbs is continually preserved by _inheritance_, although the external form is extremely changed by _adaptation_. The bones of the skeleton of the hand are drawn in white lines on the brown flesh and skin which surrounds them. All the nine hands are represented in the same position, namely the wrist (where the arm would be joined to it) is placed above, whilst the ends of the fingers or toes are turned downwards. The thumb, or the first (large) fore-toe is on the left in every figure; the little finger, or fifth toe is to the right at the edge of the hand. Each hand consists of three parts, namely (i.) the _wrist_ (carpus), composed of two cross rows of short bones (at the upper side of the hand); (ii.) the _mid-hand_ (metacarpus), composed of five long and strong bones (marked in the centre of the hand by the numbers 1-5); and (iii.) the five _fingers_, or _fore toes_ (digiti), every one of which again consists of several (mostly from two to three), _toe-pieces_, or _phalanges_. The hand of _man_ (Fig. 1), in regard to its entire formation, stands midway between that of the two large human apes, namely, that of the _gorilla_ (Fig. 2), and that of the _orang_ (Fig. 3). The fore paw of the _dog_ (Fig. 4), is more different, and the hand or breast fin of the _seal_ (Fig. 5) still more so. The adaptation of the hand to the movement of swimming, and its transformation into a fin for steering, is still more complete in the _dolphin_ (Ziphius, Fig. 6). The extended fingers and bones of the central hand here have remained short and strong in the swimming membrane, but they have become extremely long and thin in the _bat_ (Fig. 7), where the hand has developed into a wing.

The extreme opposite of the latter formation is the hand of the _mole_ (Fig. 8), which has acquired a powerful spade-like form for digging, with fingers which have become extremely short and thick. What is far more like the human hand than these latter forms, is the fore paw of the lowest and most imperfect of all mammals, the Australian _beaked animal_ (Ornithorhynchus, Fig. 9), which in its whole structure stands nearer to the common, extinct, primary form of mammalia, than any known species.

Hence man differs less in the formation of the hand from this common primary form than from the bat, mole, dolphin, seal, and many other mammals.

PLATE V. (_Between pages 84 and 85, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Vegetable Kingdom_, representing the hypothesis of the common derivation of all plants, and the historical development of the different groups of plants during the palaeontological periods of the earth's history. The horizontal lines denote the different smaller and larger periods of the organic history of the earth (which are spoken of in vol. ii. p. 14), and during which the strata containing fossils were deposited. The vertical lines separate the different main-cla.s.ses and cla.s.ses of the vegetable kingdom from one another. The arboriform and branching lines indicate, in an approximate manner, by their greater or less number and thickness, the greater or less degree of development, differentiation, and perfecting which each cla.s.s probably attained in each geological period. (Compare vol. ii. pp. 82, 83.)

PLATE VI. (_Between pages 130 and 131, Vol. II._)

_Monophyletic, or One-rooted Pedigree of the Animal Kingdom_, representing the _historical growth of the six animal tribes_ during the palaeontological periods of the organic history of the earth. The horizontal lines _g h_, _i k_, _l m_, and _n o_ divide the five large periods of the organic history of the earth one from another. The field _g a b h_ comprises the archilithic, the field _i g h k_, the palaeolithic, the field _l i k m_ the mesolithic, and the field _n l o m_ the cenolithic period. The short, anthropolithic period is indicated by the line _n o_. (Compare vol. ii. p. 14.) The height of the separate fields corresponds with the relative length of the periods indicated by them, as they may approximately be estimated from the relative thickness of the neptunic strata deposited between them. (Compare vol. ii. p. 22.) The archilithic and primordial period alone, during which the Laurentian, Cambrian, and Silurian strata were deposited, was probably considerably longer than the four subsequent periods taken together.

(Compare vol. ii. pp. 10, 20.) In all probability the two tribes of worms and Zoophytes attained their full development during the mid-primordial period (in the Cambrian system); the star-fishes and molluscs probably somewhat later (in the Silurian system); whereas the articulata and vertebrata are still increasing in variety and perfection.

PLATE VII. (_Between pages 146 and 147, Vol. II._)

_Group of Animal-Trees_ (_Zoophytes, or Clenterata_) _in the Mediterranean_. On the upper half of the plate is a swarm of swimming medusae and ctenophora; on the lower half a few bunches of corals and hydroid polyps adhering to the bottom of the sea. (Compare the system of Zoophytes, vol. ii. p. 132, and on the opposite page their pedigree.) Among the adhering Zoophytes at the bottom of the ocean there is, below on the right hand, a large coral-colony (1), which is closely akin to the red precious coral (Eucorallium), and like the latter belongs to the group of corals with eight rays (Octocoralla Gorgonida); the single individuals (or persons) of the branching stock have the form of a star with eight rays, consisting of eight tentacles, which surround the mouth. (Octocoralla, vol. ii. p. 143.) Directly below and in front of it (quite below on the right), is a small bush of hydroid polyps (2), belonging to the group of bell-polyps, or Campanulariae (vol. ii. p.

146). A larger stock of hydroid polyps (3), belonging to the group of tube-polyps, or Tubullariae, rises, to the left, on the opposite side, with its long thin branches. At its base is spread a stock of silicious sponges (Halichondria) (4), with short, finger-shaped branches (vol.

ii. p. 139). Behind it, below on the left (5), is a very large marine rose (Actinia), a single individual from the cla.s.s of six-rayed corals (Hexacoralla, vol. ii. p. 143). Its low, cylindrical body has a crown of very numerous and large leaf-shaped tentacles. Below, in the centre of the ground (6), is a sea-anemone (Cereanthus) from the group of fourfold corals (Tetracoralla). Lastly, on a small hill on the bottom of the sea, there rises, on the right above the corals (1) a cup-polyp (Lucernaria), as the representative of the stalked-jellies.

(Podactinaria, or Calycozoa, vol. ii. p. 144.) Its cup-shaped, stalked body (7) has eight globular cl.u.s.ters of small, knotted tentacles on its rim.

Among the _swimming Zoophytes_ which occupy the upper half of Plate VII., the hydromedusae are especially remarkable, on account of their alteration of generation. (Compare vol. i. p. 206.) Directly above the Lucernaria (7) floats a small tiara jelly (Oceania), whose bell-shaped body has a process like a dome, the form of a papal tiara (8). From the opening of the bell there hangs a wreath of very fine and long tentacles. This Oceania is the offspring of a tube-polyp, resembling the adhering Tubularia below on the left (3). Beside this latter, on the left, swims a large but very delicate hair-jelly (aequorea). Its disc-shaped, slightly arched body is just drawing itself together, and pressing water out of the cavity of the cup lying below (9). The numerous, long, and fine hair-like tentacles which hang down from the rim of the cup are drawn by the ejected water into a conical bunch, which towards the centre turns upwards like a collar, and is thrown into folds. Above, in the middle of the cavity of the cup, hangs the stomach, the mouth of which is surrounded by four lobes. This aequorea is derived from a small bell-polyp, resembling the Campanularia (2). The small, slightly arched cap-jelly (Eucope), swimming above in the centre (10), is likewise derived from a similar bell-polyp. In these three last cases (8, 9, 10), as in the majority of the hydromedusae, the alternation of generation consists in the freely swimming medusa (8, 9, 10), arising by the formation of buds (therefore by non-s.e.xual generation, vol. i. p.

192), from adhering hydroid polyps (2, 3). These latter, however, originate out of the fructified eggs of the medusae (therefore by s.e.xual generation, vol. i. p. 195). Hence the non-s.e.xual, adhering generation of polyps (I., III., V., etc.) regularly alternates with the s.e.xual, freely swimming generation of medusae (II., IV., VI., etc.). This alteration of generation can only be explained by the Theory of Descent.

The same remark applies to a kindred form of propagation, which is still more remarkable, and which I discovered in 1864, near Nice, in the Elephant-jellies (Geryonida), and called _allogony_, or _allogenesis_.

In this case two completely distinct forms of medusa are descended from one another; the larger and more highly developed generation (11), Geryonia, or Carmarina, is six-rayed, with six foliated s.e.xual organs, and six very movable marginal filaments. From the centre of its bell-shaped cup, like the tongue of a bell, hangs a long proboscis, at the end of which is the opening of the mouth and stomach. In the cavity of the stomach is a long, tongue-shaped bunch of buds (which on Plate VII. (_n_) is extended from the mouth on the left like a tongue). On this tongue, when the Geryonia is s.e.xually ripe, there bud a number of small medusae. They are, however, not Geryoniae, but belong to an entirely distinct but very different form of medusa, namely, to the genus Cunina, of the family of the _aeginida_. This Cunina (12) is very differently constructed; it has a flat, semi-globular cup without proboscis, consists in early life of six divisions, later of sixteen, and has sixteen bag-shaped s.e.xual organs, and sixteen short, stiff, and strongly curved tentacles. A further explanation of this wonderful allogenesis may be found in my ”Contributions to the Natural History of the Hydromedusae.” (_Leipzig_, Englemann, 1865), the first part of which contains a monograph of the Elephant-jellies, or Geryonida, ill.u.s.trated by six copper-plates.