Narrative of the surveying voyages of His Majesty's Ships Adventure and Beagle, Vol. 3 [of 3] · Charles Darwin
FALKLAND ISLANDS.
Two kinds of geese frequent the Falklands. The upland species (_Anas leucoptera_) is common in pairs, and in small flocks, throughout the island. They do not migrate, but build on the small outlying islets. This is supposed to be from fear of the foxes: and it is perhaps from the same cause that these birds, though very tame by day, are shy and wild in the dusk of the evening. They live entirely on vegetable matter. The rock-goose, so called from living exclusively on the sea-beach (_Anas antarctica_), is common both here and on the west coast of America, as far north as Chile. In the deep and retired channels of Tierra del Fuego, the snow-white gander, invariably accompanied by his darker consort, and standing close by each other on some distant rocky point, is a common feature in the landscape. In these islands a great loggerheaded duck or goose (_Anas brachyptera_), which sometimes weighs twenty-two pounds, is very abundant. These birds were in former days called, from their extraordinary manner of paddling and splashing upon the water, race-horses; but now they are named, much more appropriately, steamers. Their wings are too small and weak to allow of flight, but by their aid, partly swimming and partly flapping the surface of the water, they move very quickly. The manner is something like that by which the common house-duck escapes when pursued by a dog; but I am nearly sure that the steamer moves its wings alternately, instead of both together, as in other birds. These clumsy, loggerheaded ducks make such a noise and splashing, that the effect is exceedingly curious.
Thus we find in South America three birds, which use their wings for other purposes besides flight: the penguin as fins, the steamer as paddles, and the ostrich as sails to a vessel. The steamer is able to dive only a very short distance. It feeds entirely on shell-fish from the kelp and tidal rocks; hence the beak and head, for the purpose of breaking them, are surprisingly heavy and strong. So strong is the head, that I have scarcely been able to fracture it with my geological hammer; and all our sportsmen soon discovered how tenacious these birds were of life. When pluming themselves in the evening in a flock, they make the same odd mixture of sounds which bullfrogs do within the Tropics. In Tierra del Fuego, as well as at the Falkland Islands, I made many observations on the lower marine animals,[10] but they are of little general interest. I will only mention one class of facts, relating to certain zoophytes in the more highly organized division of that class. Several genera (_flustra_, _eschara_, _cellaria_, _crisia_, and others) agree in having singular moveable organs, like those of _Flustra avicularia_ (found in the European seas), attached to their cells. The organ, in the greater number of cases, very closely resembles the head of a vulture; but the lower mandible can be opened much wider, so as to form even a straight line with the upper. The head itself possesses considerable powers of movement, by means of a short neck. In one zoophyte the head itself was fixed, but the lower jaw free: in another it was replaced by a triangular hood, with a beautifully-fitted trap-door, which evidently answered to the lower mandible. A species of stony eschara had a structure somewhat similar. In the greater number of species, each shell was provided with one head, but in others each had two.
The young cells at the end of the branches necessarily contained quite immature polypi, yet the vulture-heads attached to them, though small, were in every respect perfect. When the polypus was removed by a needle from any of the cells, these organs did not appear in the least affected. When one of the latter was cut off from a cell, the lower mandible retained its power of opening and closing. Perhaps the most singular part of their structure is, that when there were more rows of cells than two, both in a Flustra and an Eschara, the central cells were furnished with these appendages, of only one-fourth the size of the lateral ones. Their movements varied according to the species:--in some I never saw the least motion; while others, with the lower mandible generally wide open, oscillated backwards and forwards at the rate of about five seconds each turn; others moved rapidly and by starts. When touched with a needle the beak generally seized the point so firmly, that the whole branch might be shaken. These bodies have no relation whatever with the production of the gemmules. I could not trace any connexion between them and the polypus. From their formation being completed before that of the latter; from the independence of their movements; from the difference of their size in different parts of the branch; I have little doubt that in their functions they are related rather to the axis than to any of the polypi. In a similar manner, the fleshy appendage at the extremity of the sea-pen forms part of the zoophyte as a whole, as much as the roots of a tree do of the whole and not of the individual buds. Without doubt this is a very curious variation in the structure of a zoophyte: for the growing part in most other cases does not manifest the least irritability or power of movement.
I will mention one other kind of structure quite as anomalous. A small and elegant Crisia is furnished, at the corner of each cell, with a long and slightly-curved bristle, which is fixed at the lower end by a joint. It terminates in the finest point, and has its outer or convex side serrated with delicate teeth or notches. Having placed a _small_ piece of a branch under the microscope, I was exceedingly surprised to see it suddenly start from the field of vision by the movement of these bristles, which acted as oars. Irritation generally produced this motion, but not always. When the coralline was laid flat on that side from which the toothed bristles projected, they were necessarily all pressed together and entangled. This scarcely ever failed to excite a considerable movement among them, and evidently with the object of freeing themselves. In a small piece, which was taken out of water and placed on blotting-paper, the movement of these organs was clearly visible for a few seconds by the naked eye.
In the case of the vulture-heads, as well as in that of the bristles, all that were on one side of a branch, moved sometimes coinstantaneously, sometimes in regular order one after the other; at other times the organs on both sides the branch moved together; but generally all were independent of each other, and entirely so of the polypi. In the Crisia, if the bristles were excited to move by irritation in any one branch, generally the whole zoophyte was affected. In the instance where the branch started from the simultaneous movement of these appendages, we see as perfect a transmission of will as in a single animal. The case, indeed, is not different from that of the sea-pen, which when touched drew itself into the sand. I will state one other instance of uniform action, though of a very different nature, in a zoophyte[11] closely allied to Clytia, and therefore very simply organized. Having kept a large tuft of it in a basin of salt water, when it was dark I found that as often as I rubbed any part of a branch, the whole became strongly phosphorescent with a green light: I do not think I ever saw any object more beautifully so. But the remarkable circumstance was, that the flashes of light always proceeded up the branches, from the base towards the extremities.
The examination of these compound animals was always very interesting to me. What can be more remarkable than to see a plant-like body producing an egg, furnished with setæ, and having independent movements, which soon becomes fixed, branches into numberless arms, and these, though crowded with polypi, yet in some cases possessing independent organs of movement, and obeying uniform impulses of will? The polypi are frequently animals of no simple organization; and in most respects certainly are to be considered as true individuals. It is therefore more curious to observe, in the young and terminal cells, their gradual formation, from the growth of the simple horny substance of which so many zoophytes are composed. The known organization of a tree should remove all surprise at the union of many individuals together, and their relation to a common body. Indeed we might expect, according to the apparent law, that any structure which prevails in one class will be produced in a lesser degree in some others--that since so many plants are compound, so would some animals be thus constructed. It requires, however, a greater effort of reason to view a bud as an individual, than a polypus furnished with a mouth and intestines; and therefore the union does not appear so strange.
Our conception of a compound animal,[12] where in some respects the individuality of each is not completed, may be aided, by reflecting on the production of two distinct creatures by bisecting one with a knife, or where nature herself performs the task. We may consider the polypi in a zoophyte, or the buds in a tree, as cases where the division of the individual has not been completely effected. In this kind of generation, the individuals seem produced only with relation to the present time; their numbers are multiplied, but their life is not extended beyond a fixed period. By the other, and more artificial kind, through intermediate steps or ovules, the relation is kept up through successive ages. By the latter method many peculiarities, which are transmitted by the former, are obliterated, and the character of the species is limited; while on the other hand, certain peculiarities (doubtless adaptations) become hereditary and form races. We may fancy that in these two circumstances we see a step towards the final cause of the shortness of life.
Text from Project Gutenberg, public domain in the United States.