Cometes also, in Scleropus, where they take the form of short, thick, cartilaginous stalks, with two converging leaf-apicules. Among the grasses they are known under the form of bristles in Setaria. In many Rhamnaceous and Sapindaceous plants (Helinus, Cardiospermum) they appear as small cirrhi, not as the last sterile ramifications of the inflorescence, but on the contrary as the first, followed by other fertile peduncles. They often occur in the axils of foliaceous leaves; and wherever they make their appearance they naturally arrest the farther succession of shoots, when they have neither of the two leaves at their origin, out of whose axil an additional shoot may be developed. This is the case in Passiflora, whose flower arises from the axil of a leaf situated at the side of the base of the tendril. The thorns of Ononis, Elæagnus and Maclurat present the same phenomenon. In other cases the succession of generation thus arrested by the aculeate shoot is restored by secondary formations; when, with the thorn, a second shoot follows out of the axil, which in some cases may form a leaf-shoot, and in others a flower-shoot. This happens in Gleditschia, in several Acacia (e. g., A. pulchella), in Prinsepia utilis, the Lemon, the Egyptian Balanites, Duranta, Bouganvillea and Randia, in which the secondary shoot arises close under the spine; while in Celastrus pyrrhacantha and Europeus, as well as Pisonia aculeata,|| the secondary shoot occurs above the thorn. In Uncaria pilosa¶ and Strychnos spinosa, pairs of leaves with axillary thorns alternate with pairs which have peduncles in their axils. Have even these phenomena of extreme alienation of the individual (as they occur in the thorns and hardened shoots of plants) analogous forms in the animal kingdom? Yes, I believe they have! I believe I may assert that in the animal kingdom itself there are individuals which occur as mere fixed claws, pincers, Scourges, tactual and predial filaments, etc.,-individuals which perform neither functions of nutrition nor of reproduction in the society to which they belong, but which probably merely assist in seizing the food, or lend a helping hand in defending the community. The cases which I have here in mind are of frequent occurrence among Bryozoa, and especially in the group The plumose tails which form the "envelope" of Cometes, are the last branches of the dichotomous inflorescence, accompanied by similar accessory (secondary and tertiary) branchlets. All these numerous sterile branchlets are elongated and beset with setiform leaflets arranged in spiral order (2), commencing with two similar anterior leaves. The direction of the phyllotaxis in all these branchlets follows the law of furcate inflorescence. Here belongs also the curious hook of Uncinia, which is also visible, though less developed, in many species of Carex. The utriculus is a leaf at the base of this spine. Boyle Illustr. of the Bot. of Himal., pl. 38, fig. 1. Wallich: Plant. As. rar., t. 170. of Cellaria. Individuals in the form of horns (which usually conclude the series of complete cell-inhabiting individuals) occur, e. g., in Eucrabea cornuta,* and Cordierii;† in another form (reminding us of Teloxys,) as forked terminal spines, in Vesicularia spinosa. Moveable individuals, representing mere weapons, in form like a bird's beak, a crab's claw or a pincers, appear in Acamarchis avicularias and flustroides,|| Retepora cellulosa Scrupocellaria scruposa¶ and many others. In the last named Cellaria, besides the claw-individuals, there are also scourge-individuals, which Van Beneden himself compared to the cirrhi in plants, and which even Leuckardt** acknowledges to be individuals. Beside the Swimming-bells' evidently resembling Medusa, the peculiar retractile predial filaments of the Siphonophore doubtless belong here also; they are remarkable for a purplish-red swelling on or under the apex, and they shoot out singly as branches from the stalk of the nutritive individual (imbibing-tubes), and themselves bear a series of similarly formed filaments as secondary branches. They are found with unimportant departures from this form, especially in Physophora,†† Diphyes‡‡ and Agalmopsis. In the last named genus, according to Sars, they have even three modifications: the spadiciferous terminal piece ends in a long simple filament, or in a short twoparted oue, or without any filament at all. In Stephanomia|||| numerous filaments, called tentacles, arise out of the stalk of the nutritive animals (the so-called proboscis-formed organs) without such colored swellings, which in the same manner may also be regarded merely as individuals with a very incomplete outfit of organs.¶¶ * Ellis: op. cit. pl. 21, f. 10. (Cellaria cornuta); M. Edw.: Ann. d. Sc. Nat., (1838) t. 8, f. 2 (Crisidia cornuta). + Descrip. de l'Eypte: Polypes, t. 13, f. 3. Crisia avicularia Van Beneden : 1. c., t. 6, f. 1-8 (Cellularia avicularia Pall. Lamx.) Ellis: op. cit., pl. 38, f. 7. ☛ Van Beneden: 1. c., t. 5, f. 8--16 (Cellaria scruposa Auct.) **Leuckardt: Polymorphism. p. 17. tt Philippi: Müller's Archiv, 1843, taf. 5. §§ Ib. tab. 5. Sars: Fauna lit. Norw. tab. 7. Milne Edwards: Ann. d. Sc. Nat., 1841, pl. 7--10. Since Sars observed the separation of the Medusa-like sexual individuals in Agalmopsis, the view that Siphonophora are composite animal stocks has gained ground more and more among zoologists. But this mode of viewing the subject was for the first time carried out (after a fashion,), consequently in Leuckhardt's latest work on strange animal forms (Zool. Unters, erstes Heft: Siphonophoren, 1853); and this idea had forced itself upon me as early as 1847, when I compared the description Diphyes with Agalmopsis, in Sars' Fauna lit. Norw. In the above named work, Leuckhardt extends the view which allows individual importance to the parts of the stock of Siphonophora not only to the tentacles and predial filaments, but also to the covercles, which in most of the genera are placed close above the nutritive individual as protective envelopes; these formations, like all the other appendages of individual importance, being emitted from the stem as shootlets, and in the first stages of their formation, resemble the tentacles in particular. Accordingly Siphonophora have not less than eight different forms under which the individual may appear on the whole stock. (Later note.) [I have omitted the enumeration of these forms.-T.] After having in the foregoing review regarded all lateral shoots which spring from the main axis of the plant as real individuals, however unimportant a fraction of the total specific character they may realize, it will hardly be deemed surprising if we finally apply this mode of view to the branches of the root and to adventitious shoots. It is only possible for the main-shoot to develop freely both the points of vegetation of the axis; yet even here the lower point remains undeveloped. On the contrary, the lateral shoots, thus far considered, have no lower point of vegetation; for their base is united to the maternal shoot, and hence they are mere developments of the upper point of vegetation. Opposed to these, there are, however, other shoots by which the lower point of vegetation is represented, and which on the other hand have no upper point of vegetation. Among these may be reckoned not only the root-branches which take their rise from the main root, but also all adventitious roots which spring from the stem at determinate or indeterminate places. I must, however, content myself with this general hint, as any attempt to particularize these relations could after all only show the deficiency of the investigations into this subject, and how desirable a more comprehensive work is on root-formation in the vegetable kingdom. The few points which I have selected out of the inexhaustible field of shoot-formation in the vegetable kingdom may in the mean time suffice to show that the comparison of the vegetable shoot with the animal individual is not far-fetched or arbitrary, but is presented to us by Nature herself. The solution of the difficulties which this mode of conceiving the vegetable individual encounters in the lowest grades of the vegetable kingdom, I must defer to a later day. These difficulties are founded upon the less complete organization of the inferior plants, and at all events cannot invalidate the results gained in considering the higher organizations. We may therefore consider it settled, that although the individual has not exactly the same importance in the vegetable kingdom as in the animal, plants still realize their vital cycle in sections which are not only comparable to the animal individual, but are in fact its complete analogues. What distinguishes plants is the formation of family-stocks, (a formation manifested in the highest vegetable representations, and here in the richest fullness),—as ancestral trees organically connected, variously disposed in their ramifications, and comprising numerous generations, rendered reciprocally complete through individuals variously endowed. And this leads us back again to the tree from which we set out; in which even our natural perceptions seemed to discern something more than one common individual, and whose high import scientific research must confirm. Just what at the outset appeared to be an obstacle to our allowing the single shoots of the tree their true significance,-now that we have compared them with alternation of generation in animals at length proves to be the most conclusive demonstration of the correctness of our first conception. The conception of these so heterogeneous shoots as individuals of one and the same species has led us, in fact, to a more profound and more pregnant conception of individuality, which will no longer seem paradoxical when we perceive it is confirmed even in the highest realms of life-in the sphere of the mental development of the individual. Or are the differences of human individuals in mental endowment and development less important than those which we have seen in the morphological and physiological endowment and development of shoots? Do we not meet with a similar reciprocal completion, a similar division of labor among the individuals of the family, of the state and of nations, and cannot even the human individual become likewise a mere organ? Do we not see the development of the human race itself bound up with a succession, in which the later generations continue the edifice their predecessors began, like branches depending upon the earlier stocks and nourished by them;-in which generation is added to generation, and cycles to cycles; so that thus by the ever-renewed labor of the individual the problem of human life may be ceaselessly aspired to, and at last reach its final accomplishment?* * The preceding pages were almost all printed when I was fortunately enabled to read Reichert's memoir (die monogene Fortpflanzung, Dorpat, 1852,) upon a subject closely allied to the one here discussed. His work is full of new views of the subject, elaborated with great acuteness. The vegetable individual itself is considered in detail, and the author is thus led to a mode of viewing this subject similar to the Schultz-Schultzenstein-ian doctrine of anaphyta-regarding not only the shoot, but even its single parts, the internodes, with their leaves, as series of individuals shooting out of each other, or intimately connected by continuable bud-formation. Since, however, it is implied in the idea of an individual, that it shall somehow be limited by, and distinguishable from, (notwithstanding it is connected with) others; it seems to me that even from this point of view Reichert's idea can by no means be carried out. I will not deny that there are still other considerations in the nature of the shoot which it is difficult to reconcile with the idea of the simple individual, and I can only find the ground of this phenomenon in the fact, that the individual appears in its full import in the higher steps of the series of created beings, while in the lower it loses more and more its reality, if I may so say. I must reserve farther remarks on this subject until I treat of the individuality of the lower plants. [We cannot but think, after all, that this view of Reichert's, &c., which our author rejects, is the legitimate conclusion, to which the very line of argument so completely and ably presented in the preceding pages, when fully carried out, naturally leads. It is merely a question of degree of individuality. As yet, perhaps, no sure middle ground has been secured between the two extreme views,—one of which regards all the vegetative offspring of a seed, however numerously multiplied, as philosophically the individual; while the other views the phyton, or in the simplest lower plants the cell, as philosophically representing the individual,-real individuality being incompletely realized (and with various grades of incompleteness) in all vegetables, and in many animals. The mind is reluctant to accept either of these conclusions, and seeks-thus far in vain-for some stable intermediate view. Of the two extreme views, if forced to the choice, we should incline to prefer the latter.-A. G.] ART. X.-Observations on Binocular Vision; by Professor WILLIAM B. ROGERS. PART THIRD Of successive or alternating combinations of lines. WHEN the figures presented to the two eyes consist of lines capable of being united in two or more different ways these combinations may be produced successively by a voluntary change of convergence, and in certain cases they are observed to follow one another in quick alternation without our being conscious of effort in producing them. 21. Alternation of Vertical lines. The simplest example of this effect occurs when a figure composed of three equal verticals is so placed in the stereoscope that we may unite one of the extreme lines with either of the other lines successively. a 45. Thus placing fig. 45 on the upper stage of the instrument so that a may be in front of the left eye while b and c are both presented to the right eye, we may combine a with b; and keeping the eyes directed to their resultant, we at the same time observe the line c a little to the right. Now changing the optic convergence to a point somewhat more remote we can cause a to quit b and to pass more or less rapidly over to c leaving b alone on the b left. By thus changing the convergences backwards and forwards we may continue to unite a alternately with b and c with but little effort and as often as we please. When the distance between b and c is very small, say one twentieth of an inch, this change seems to me almost involuntary and occurs with the rapidity of a flash whenever we transfer our attention from the resultant ab to the simple line c, or back again from the resultant a c to b. Indeed, as we view the resultant and the parallel beside it, the line a is seen as it were to flit backwards and forwards between b and c, and it is only by fixing the attention resolutely on the resultant that we can prevent the alternate decomposition and recomposition of the lines. A dot placed a little above the line a, by accompanying the line in its movements, enables us to mark its successive union with b and c. 22. Alternation of Vertical with Oblique lines. Still more curious illustrations of alternating combinations are presented when the picture includes one or more inclined lines as in fig. 46. Placing this on the upper stage of the stereoscope so that a shall be in front of the left, and b and c in front of the |
