« AnteriorContinuar »
survival of the fittest, must appear to us superfluous laws of nature.”
We cited this passage and remarked upon it when it was first published in Mr. Darwin's “Plants and Animals under Domestication.” His argument simply reminds us of a difficulty not at all peculiar to natural history or physiology, but which encounters us in all directions. Evidently it is not the plan of nature to reach what we call good ends, without what look like breaks, interruptions, and failures. If speculations on the modifications of organic beings according to the principles of Mr. Darwin, bring us into contact with many fresh puzzles and perplexities of this description, they also supply a fresh store of facts, which tend to increase our belief that the system is conformable to our religious instincts and moral nature. No natural theologian can affirm that any theory yet propounded, supplies a satisfactory explanation of all the moral difficulties, or intellectual difficulties which stand in the way of a perfect comprehension of the character of the great plan. Why it is obviously benevolent in a thousand directions, and apparently harsh in a thousand others, we do not know, any more from Darwin than we did from Paley, but we certainly are not left in a denser
and as modern researches have enabled us to catch glimpses of a far wider, more complicated, and comprehensive plan than the older thinkers had any conception of; we may, while lamenting the limitations of our mental vision, take comfort in the belief that in the vast regions of the yet unknown, there lie ample satisfaction for all our hopes, and ample resolution of all our doubt. Did modern science narrow the aspect of the natural plan, doubt would grow and faith decrease, but when we are led to consider what may be the requirements of a system uniform in character, extending through all time, and comprehending all space, there is no wonder that we are lost in any efforts to solve the ultimate problem of the whole.
We are only entitled to ask of each science to unfold its own particular truth. The naturalist and the physiologist or biologist speculatiug on the origin of life and species, are bound to apply a strictly logical and exact method. They have nothing to do with the suggestions of the imagination, except to test them by comparison with fact; or with the promptings of the emotions and feelings, except to prevent their leading reason and accurate deduction astray. Man must not delude himself by overstating what he knows, or making what he wishes the measure of what he pretends to understand. Even when observation, experiment, and reason have done their work,
VOL. III.-NO. IV.
he must not expect that they can make all clear. Were the observation large enough, and the reason wide enough, all darkness might disappear, but explorations of nature by short-lived travellers, with feeble faculties, bound to a small spot on one small globe, can yield no conclusive interpretation, but may help us to get nearer to the Source of Light, if we allow conviction and belief to be successively modified as more facts are understood, and let the battle of opinions be freely fought out, until the survival of the fittest terminates the strife.
BRITISH HYDROID ZOOPHYTES.*
The season for sea-side visits has commenced, and those who seek our shores with some better purpose than the cultivation of frivolity, or the display of costume, can have their attention directed to no more beautiful objects than the British Hydroid Polyps, which have found a worthy historian in Mr. Thomas Hincks. Johnstone's well. known “ History of British Zoophytes” was an admirable production for the period in which it was written, and must retain a prominent place in the library of the naturalist. It was, however, his function to treat of a branch of science in a very incomplete state, and his labours had the merit of materially contributing to those further researches and discoveries, which have rendered it necessary that an entirely new treatise should be composed. Mr. Hincks was eminently fitted for this task, uniting an enthusiastic love of his subject with accuracy of observation, and remarkably elegant and lucid description.
Making himself familiar with the researches of other naturalists, enjoying the assistance of many of the most distinguished observers, and availing himself of frequent opportunities of studying the living forms in their native localities, he has been able to bring together & great body of information, and, for the first time, to produce a satisfactory monograph of a very important group, and his book will take its place in the front rank of natural history authorities. It is always a source of great pleasure when we can recommend a scientific work as combining exactitude with literary merit and simplicity of style, and Mr. Hincks deserves no small praise for the care with which he has avoided one of the besettivg sins of our
*"A History of the British Hydroid Zooyhytes." By Thomas Hincks, B.A: 2 rols. Van Voorst.
time, the endless multiplication and employment of hard words. Almost every author seems to fancy it his duty to invent a fresh set of alarming compounds from Latin and Greek, often unskilfully constructed and clumsily applied. As soon as a new and unknown object is seen, dreadful nicknames are invented by the dozen to describe its structure and peculiarities. Its real nature, life history, and affinities, may be all undiscovered, but such trifling facts oppose no obstacle to the mania for long-tailed word making. Larval forms, transition forms, and adult forms all have names given to them before the relations in which they stand to each other are known, and when the student has been plagued with a host of terms manufactured without foresight, the whole lot will be found inappropriate, and new writers retain some, invent others, and leave to their followers, again, the task of devising fresh appellations to suit new theories or new facts. An inevitable result of this craze for hard words is, that their meaning becomes uncertain, fluctuating, or vague, and it is much more difficult for the student to learn the names than the properties of the things they are intended to describe.
Mr. Hincks has written his book so simply as to minimize difficulties of this description. He has only employed technical terms when common words would not suffice; he has clearly defined those which he has used, and thus rendered his book a pleasant companion as well as an instructive guide.
The hydroid zoophytes take their name from the common polyp, —the hydra, well known to all microscopists and naturalists,-and they offer a singularly fascinating group for sea-side study, not only from the remarkable beauty of form and colour which many of them exhibit, but likewise for the very curious and interesting physiological problems they present. In the hydra we have a solitary animal capable of locomotion, budding off young ones exactly like itself, and at certain seasons combining sexual elements, forming true eggs, and giving rise to distinct individuals. It is common to see the mother hydra with her budded infant still attached, but this connection soon ceases, and nothing at all resembling a permanent colony is formed.
Amongst the hydroid zoophytes solitary life is the exception, and the rule is for a colony to be formed by a process remarkably resembling vegetative growth. A young polyp, having passed through larval stages, usually settles down, forms a sort of creeping root (stolon), and buds off, one after another, a series of polyps like itself, the whole in many families aggregated together like the
branches of a tree, of which the polyp heads are the flowers. The hydroid colony is composed of a common fleshy substance, the conosarc, of a tubular form, composed of an outer and inner layer, the former very commonly hardened into a protecting sheath. The polyps, or polypites, grow like branches from the cænosarc as a stem. Sometimes they form a naked projection from the end of a tube; at others this tube is terminated by a graceful cup or urn, in which the polyp lives, thrusting forth its tentacles and part of its body at will, and returning to its home for shelter when wearied or alarmed. Some nervous connection between all the members of the colony must exist, though no special nerve-fibres are observed. The whole colony frequently acts from a common impulse, though each member of it has an individual life and will. The tentacles of the polyps are richly furnished with curious organs called thread cells, out of which dart minute threads simple or armed, and possessing remarkable properties of poisoning and paralysing their living prey. Each polypite or member of a polyp colony, in addition to feeding himself, contributes to the nutrition of the compound whole, sending streams of digested particles from its stomach through the common tube. Thousands of polypites are associated together in many species, and it is not uncommon for them to die off like flowers, and for their successors to be reproduced by new buds from the tree-like stem.
Illustrations of these facts may be easily obtained by any visitor to our coasts, as the tree-like zoophytes abound on or under rocks uncovered at low water, occur frequently on shells brought out by the fishermen, and are recognized with little difficulty by any intelligent observer.
The most marvellous part of hydroid zoophyte life is connected with its reproduction by true eggs. After a sufficient course of budding, the egg formation takes place. Buds appear in which ova and sperm-cells are developed. The contents of the sperm-buds reach the germ-buds, and a new race begins. In the hydra these two kinds of buds grow on different parts of the same individual; but in a great many cases the function of sexual reproduction is consigned to a special class of zooids, which leave the colony and become free swimmers in the shape of jelly-fish or medusæ. The hydroid colony in these cases exhibits at certain seasons two sorts of individuals : the polypites, which are its food collecters and digestors, and the sexual zooids, which are to found new colonies by forming and maturing eggs.
The sexual zooids are either fixed or free: the latter possessing
especial interest. No one could have expected that a tree-like colony of polyps would bud off at certain seasons bell-like jelly-fish or medusæ, and it is no wonder that the latter were long considered as belonging to quite another race. “So thoroughly,” says Mr. Hincks, “are their affinities concealed by the locomotive and adaptive organs with which they are furnished, that they might readily pass for members of another tribe. Indeed, we can scarcely imagine a more complete contrast to the staid and stationary zoophytes, in outward form and habit of life, than the medusiform zooid, which it evolves from its own substance, and sends forth with the seed of new commonwealths.” These objects are easily collected in calın weather by the tow-net at the sea-side, and the smaller ones are amongst the most beautiful things the microscope can display.
“ Gemmation,” we quote Mr. Hincks, “is not confined to the fixed portions of the hydroid colony; it also enters into the history of the free and locomotive zooids. In many cases they manifest the vegetative tendencies of their tribe, and multiply rapidly by budding. Gemmation seems usually to take place when the true reproductive function is in abeyance. Thus in the spring the gonozooid (sexual zooid) of Clavatella develops buds on the margin of the body between each pair of tentacles, which are cast off at certain stages of growth, while later on in the year the vegetative activity ceases, and reproduction by ova and spermatozoa takes its place. These buds, which are analogous to those produced by the hydra, bear an exact resemblance when mature to the zooid that originated them. ... The free zooid after its detachment, may pass through many stages of growth and development itself, and originate a large number of similar organisms, before proceeding to discharge its principal functions, the elaboration of the generative elements; with the escape and dispersion of the latter, its existence, in all probability, usually terminates.”
Although at first sight the medusa and the polyp seem different, resemblances are found on careful examination ; " The free sexual zooid, in all but one or two exceptional cases, may be regarded as essentially a polypite, with its arms united by a contractile web, so as to form a floral and natatory organ. Disguised by its adoptive dress, it has been separated from its kindred under the name of a medusa; it is in reality a swimming polypite. When liberated it matures, and disperses the generative elements, and having thus fulfilled its function, perishes.
“In other cases the gonozooids never become free, but remain in