Science

READERS who were interested some years ago in Dr. Forbes Winslow’s little book on the “ Physiological Influence of Light,” would do well to read General Pleasonton’s accounts of his experiments on “ The Influence of the Blue Color of the Sky in developing Animal and Vegetable Life.” For more than ten years, General Pleasonton has been engaged in experiments which have consisted in subjecting growing plants and animals to an artificially increased intensity of the violet rays of the spectrum, or those rays which lie nearest to the invisible actinic rays. Cuttings of grape-vines, for example, one year old, were placed under a roof in which every eighth row of panes was violet-colored; and under these circumstances the growth of the vines went on with astonishing rapidity. At the end of five months many vines had attained a length of more than forty-five feet. In the second season, not only was the growth even more rapid than this, but in addition the vines were heavily loaded with enormous bunches of very largesized and healthy grapes. Now, after ten years, the vines are still growing luxuriantly, and though they have borne immense crops, sometimes estimated at more than two tons, without intermission since their first crop, they as yet show no signs of old age. Of especial interest is the fact that the incessant production of such large crops has not interfered with the regular formation of woody fibre or with the growth of a dense foliage. It is a general law, in the vegetable as well as in in the animal kingdom, that the exercise of the reproductive function is a serious hindrance to the growth or development of the individual ; and the fact that, under an increased supply of violet light, a plant will continue to grow vapidly even while it is ineessantly producing large numbers of seeds, shows most strikingly to what an extent its vital energy has been increased.

In 1869, General Pleasonton proceeded to experiment upon the effects of the violet ray in stimulating animal life. Out of a litter of eight young pigs he placed four in a piggery containing violet glass in three sides and in the roof, while the other four were placed in a piggery exactly similar in construction but containing ordinary white glass instead of the violet. At the end of six months, during which both sets of pigs had been supplied with exactly the same quantities of food, and subjected in general to the same kind of treatment, the superior growth of the pigs kept under the violet glass had become very remarkable. In similar wise, by furnishing an extra supply of violet rays, General Pleasonton has caused a sickly and puny bull-calf to develop into an animal of magnificent size and strength. It is to be hoped that these interesting experiments will be continued on a still larger scale.

IN a recent address before the Entomological Society of London, Mr. Wallace calls attention to the very ingenious and plausible hypothesis propounded some time ago by Mr. Herbert Spencer, to account for the origin of the annulose or articulated sub-kingdom of animals. According to this hypothesis any annulose animal is in reality a compound organism, each of its segments representing what was originally a distinct individual. In other words, an annulose animal is a colony or community of animals which have become integrated into an individual animal. Strong prima facie evidence of such a linear joining of individuals primevally separate is furnished by the structure of the lowest annelids. Between the successive segments there is almost complete identity, both internal and external. Each segment is physiologically an entire creature, possessing all the organs necessary for individual completeness of life; not only legs and branchiæ of its own, but also its own nerve-centres, its own reproductive organs, and frequently its own pair of eyes. In many of the intestinal worms each segment has an entire reproductive apparatus, and being hermaphrodite, constitutes a complete animal. Moreover in the development of the embryo the segments grow from one another by fission or gemmation, precisely as colonies of compound animals grow. At the outset the embryo annelid is composed of only one segment. The undifferentiated cells contained in this segment, instead of being all employed in the formation of a heterogeneous and coherent structure within the segment, as would be the case in an animal of higher type, proceed very soon to form a second segment, which, instead of separating as a new individual, remains partially attached to the first. This process may go on until hundreds of segments have been formed. Not only, moreover, does spontaneous fission occur in nearly all the orders of the annulose sub-kingdom, but it is a familiar fact that artificial fission often results in the formation of two or more independent animals. So self-sufficing are the parts, that when the common earthworm is cut in two, each half continues its life as a perfect worm. Very significant is the fact that in some genera, as in chætogaster, where the perfect individual consists of three segments, there is formed a fourth segment, which breaks off from the rest and becomes a new animal.

All these facts, together with many others of like implication, point unmistakably to the conclusion that the type of annulosa has arisen from the coalescence, in a linear series, of little spheroidal animals primevally distinct from one another. How, now, are we to explain, or to classify, such a coalescence ? Obviously, the coalescence is to be classified as a case of arrested reproduction by spontaneous fission. In other words, whereas the aboriginal annaloid had been in the habit of producing by gemmation a second individual which separated itself at a certain stage of growth, there came a time when such separation became arrested before completion; so that, instead of a series of independent organisms, the result was a colony of organisms, linked together in a linear chain. Let us observe that by this brilliant explanation the origin of the annulose type is completely assimilated to the origin of the lowest animal and vegetable types. The primordial type alike of the vegetable and of the animal, is a single spherical or spheroidal cell, which reproduces itself by spontaneous fission. That is, it elongates until room is made for a second nucleus, after which a notch appears in the cell-wall between the nuclei ; and this notch deepens until the old and new cells are quite separated from each other. Now when many such primordial cells are enclosed in a common membrane, so that, instead of achieving a complete separation, they multiply into a jelly-like or mulberry - like mass, there is formed — whether the case be taken in the animal or in the vegetable kingdom — an organism of a type considerably higher than the simple cell. There is an opportunity for differently conditioned cells comprised in the same mass to become differently modified, and thus to subserve various functions in the economy of the organism. There is a chance for division and combination of labor among the parts. Now the progress achieved when the spheroidal members of an annaloid compound remain partly connected, instead of separating, is precisely similar to this. Among the indubitably compound animals of cœleuteratc or molluscoid type, in which the fission is not arrested, it is but seldom that the individuals stand related to one another in such a way that there caft be any need of their severally performing diverse and specialized functions. For instance, among the hydrozoa, each member of the compound can get food for itself, can expand or contract its tentacles in any way without affecting the general welfare of the compound. But now, if the members of such a compound as the hypothetical primitive annaloid are grouped in a linear series, there must arise a difference between the conditions which affect the extreme members of the series and the conditions which affect the intermediate members. And consequently there will ensue an advantage to the compound in the struggle for life, if the members, instead of continuing to perform identical functions separately, become sufficiently united to allow of their performing different functions in concert. Hence we obtain the lowest actual type of annaloid, in which the segments are mere repetitions of each other, with the exception of the extreme front and rear segments, which subserve different functions related to the well-being of the aggregate.

Viewed in this light, the various great classes of the annulose sub-kingdom beautifully illustrate that progressive co-ordination of parts becoming more and more unlike one another, which is the chief characteristic of progress in the organic world. In very low annelids, such as the intestinal worms, we see hardly any specialization among the parts ; and as we proceed upwards through the lower types, ending with the myriapoda, of which the centipede is the most familiar representative, we meet with a great but varying number of segments, which show but little specialization, save in the head and tail. The same is, in general, true of the larvæ and caterpillars of the higher types. But as we rise to the adult forms of the insect group, — comprising crustaceans, arachnoids, and true insects, — we find the number of segments reduced to just twenty. And while this number remains unvarying, the modifications undergone by different segments in conformity to the requirements of the aggregate are almost endless in variety, the extremes, both of concentration and of specialization, being seen in the ant, the spider, and the crab. In many of the details of this gradual fusion of distinct individuals into a coherent whole, we see the hypothesis interestingly illustrated and justified. In the annelids of low type, each segment has its own spiracles which have no internal communication with one another. On the other hand, in the insect group there is a complete system of vessels connecting the respiratory systems. While in the intermediate myriapoda we find, as might be expected, a partial communication.

For fuller information on this subject the reader may consult Mr. Wallace’s Address, or the second volume of Mr. Spencer’s “ Principles of Biology.” We are glad that attention has again been directed to this very suggestive hypothesis, which, whether it prove adequate or not to explain all the facts of morphology in the annulose sub-kingdom, cannot fail to be of great service in the study of this branch of biology. Now that the origin of the order of insects has become such a conspicuous subject of discussion, it is time that this line of explanation should be further pursued and more thoroughly tested.

In this connection we may remark upon an apparent fallacy which occurs in M. Joachim Barraude’s recent work on “ Trilobites.” In this laborious and well-elaborated monograph, the learned author regards it as a difficulty in the way of the derivation - theory of organic forms, that many of the oldest known trilobites possess a great number of segments, while, at the same time, the embryonic forms of trilobites in general possess but few segments. And so, argues M. Barraude, there is a violation of the rule that animals, in the course of their embryonic development, should repeat the forms of their ancestors. We had supposed it to be generally understood that such repetition is hardly ever, if ever, strictly literal, but is always, or nearly always, merely approximative. Until it has been shown that all caterpillars must possess segments at least as numerous as those of the lowest known annelids, it is difficult to see what new weight can be accorded to M. Barraude’s objection.

For the rest, in spite of its rather antiquated zoölogical theorizing, we may cordially recommend M. Barraude’s monograph, which contains an excellent summary of the development of trilobites, and especially a comparative view of the occurrence of cephalopods and trilobites in the Silurian system of Bohemia.