AN INTERESTING CASE OF NATURAL SELECTION.

IN

SAMUEL F. CLARKE.

N the early part of last spring I obtained a large number of the gelatinous egg-masses of one of our native salamanders or newts (probably Amblystoma opacum). They had been deposited in a small pond of clear water, in the edge of a wood just outside the city.

These egg-masses, or bunches of eggs, vary greatly in size, the smallest being no larger than an English walnut and containing only from five to eight eggs, while the largest bunches are from six to eight inches long, more or less oval in shape, and contain from one hundred and fifty to two hundred eggs. The bunches are usually attached to some water-plant or to an overhanging blade of grass, and the gelatinous matter is so translucent that the dark, opaque eggs may readily be seen through it. Each egg is surrounded by two membranes, between which there is quite a space; and as this space, as well as that within the inner membrane, is filled with fluid, an admirable arrangement is thus secured for protecting the embryos from any injury to which they might be exposed by coming in contact with any hard, unyielding body.

The eggs were kept in large glass jars and developed quite rapidly, the rate of growth seeming to depend upon the purity and temperature of the water. After their gills and balancers were developed, they emerged from the eggs and began their active life in the water. And now I found trouble in keeping them, for I was unable to find what they wanted for food. I tried various things but did not succeed in pleasing them. Upon watching them closely I soon found that they had developed cannabalistic tendencies and were eating off one another's gills. This led me to study their movements still more closely, when I soon discovered that among the many there were a few, who although they came from the same parents and were subjected to the same conditions while in the egg, were yet gifted with greater vigor and energy than most of their brothers and sisters or cousins. These few stronger ones eat off the gills of many of the weaker ones and at the same time were enabled to protect their own from mutilation or destruction.

These favorable conditions, the large supply of food and the better aeration of the blood, soon began to show their influence upon the growth of the individuals thus favored. Within a week

or ten days from the escape from the egg, these favored few were fifty per cent. larger than their weaker comrades who were born upon the same day. Their mouths had by this time increased so much in size that they were no longer satisfied with nibbling off the gills of their brethren, but now began to swallow them bodily. This great increase in the supply of food soon produced a marked effect upon those who were thus supplied; so that in ten days from the time that they began to feed in this way they were from ten to twelve times the length and bulk of those upon whom they were feeding. Developing at this rapid rate, they arrived at the stage when the gills are re-sorbed and the abranchiate form leaves the water for the marshy land or old, damp log, where it usually makes its home and where it would find a supply of more natural food-material.

Here then was a very interesting case of natural selection, by survival of the fittest. All the weaker individuals being destroyed and actually aiding the stronger ones by serving them as food until they could pass through their changes and escape to other regions where food was more abundant.

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RECENT LITERATURE.

FLÖGEL ON THE STRUCTURE OF THE BRAIN IN DIFFERENT ORDERS OF INSECTS.-The Supplementary Heft for May 28th of Siebold and Kölliker's Zeitschrift für Wissenschaftliche Zoologie contains an elaborate article by J. H. L. Flögel, illustrated by a number of micro-photographs. This and Dietl's excellent paper, published in 1876, are the only treatises on the minute structure of the brain of insects, Owskianikof having studied that of the spiny lobster (Palinurus) several years ago, while Dietl studied the brain of Astacus. Flögel establishes three points as the results of his researches.

First, the constant presence of the remarkable central body in the mature insects of all orders, while it is almost absent in the larvæ of Lepidoptera (but not in Hymenopterous larvæ). We are thus led to suppose that it has something to do with the formation of the faceted eyes. If it has any relation with the bundle of fibres passing from the optic lobe, there is nothing to indicate it.

Secondly, the size of the olfactory lobe, with its olfactory. bodies, correlated in insects with small antennæ entirely unfit for tasting, but on the contrary with a very completely developed sense of smell, is in the author's opinion an excellent proof of the correctness of Leydig's view that the antennæ are organs of smell, whatever may be brought forward in opposition to it. If they are to be interpreted as an apparatus for detecting sounds,

we, on the other hand, are acquainted with the finer structure of the organs of hearing in the Orthoptera, and know that they have no such constituted brain-centres as the olfactory lobes.

Thirdly, Flögel draws attention to the wonderful and so little understood facts that in insects, where the lobes ("bechers" of Flögel, "lappen," "gestielte körper," etc., of Dietl) and the substance around it (gerüst) constitute the greater part of the brain, there is indeed no connection of the nerve-fibres to be found with the remaining parts of the brain, and consequently also with the œsophageal commissures. The opinion that the ganglionic cells are in direct relation through fibres with the organs of the body is provisionally unfortunately contradicted. But where are the intermediate stations? he asks.

Finally, the author claims that the essay indicates the outlines of a future brain-topography for insects, and shows that the single parts of the brain have their homologues in the different orders of insects; consequently a ground-plan in the organization is not to be mistaken, and thus a comparative anatomy of the brain of insects is outlined comparable with that of the vertebrates, as established by the researches of Stieda.

BARROIS' EMBRYOLOGY OF BRYOZOA.-The author of this elaborate and beautifully illustrated memoir is well-known for his able and thorough work on the development of the sponges and nemertean worms. A large number of typical forms of Polyzoa or Bryozoa, as the German and French call them, have been studied, and the different stages figured, including the genera Loxosoma, Pedicellina, and several genera of higher marine forms. We will give the general results of our author's work condensed from the résumé général. A study of the different groups of Chilostomatous and Cyclostomatous Polyzoa, shows that their development presents the same phenomena, characterized by the great regularity of the segmentation (morula) and giving rise to a blastula, in which the advanced morula is composed of two distinct halves (oral and aboral) separated by a crown of cilia. Then the gastrula

state is assumed and afterwards the mesoderm arises. At the moment of birth, the embryo always withdraws the aboral end within the crown and thus assumes a discoidal aspect, but it can undergo this pro

FIG. 1.-Blastula of Alcyonidium mytili. FIG. 2.Gastrula of the same; c, beginning of crown.

cess much more rapidly, previous to the appearance of the fur1 Recherches sur Embryologie des Bryozoaires. Par J. BARROIS. Lille, 1877. 4, pp. 305, with 16 plates.

row, so that we are led to distinguish in certain types, the presence a little after the gastrula stage of a stage with an aboral mass and extended or widened face separated one from the other by the furrow (Fig. 3, sb). Hence he distinguishes two fundamental forms; that which presents in the embryos the division into an aboral and extended face (Alcyonidian or pedunculate form), and that in which the furrow is wanting, constituting the Escharine or sessile form. Barrois considers that the latter is the original form, and that the Alcyonidian form (Fig. 4) is derived from it.

FIG. 3.-Larva of A. mytik.

The formation of the recurved intestine of the Polyzoa results from a closure of the opening of the digestive cavity, analogous to the closure of the blastoderm in Clepsine and in Euaxes. The Ectoprocts pass during the period of their development through an Entoproctous condition in which the part which represents the intratentacular space or basilar plate, which separates the digestive cavity from the cavity of the sheath, contains the two openings of the digestive tube, and is completely encircled by the tentacles. Barrois thinks, contrary to Allman, that it is much more natural to consider the Ectoprocta as organisms throughout comparable to Entoprocta, but in which the anus curves within the tentacular crown, as he had shown to be the case at the time when the tentacles bud out; and it is very improbable that the transitory state in which the crown is interrupted on the anal side is the point of departure of the formation of the lophopore; we shall thus have a general phylogeny of the class of Polyzoa, based on the evolution of the tentacular crown, and disposed as follows: Entoproctes-Lophopodes-Gymnolemes.

He considers that all the different larval forms of Polyzoa may be reduced to a single type composed of a gastrula with two opposite faces or ends separated by the crown, one (aboral) bearing in its centre the buccal opening, and capable of being covered so as to form the vestibule; all the larvae possess a median muscular or fatty layer (mi), which is generally composed of a portion formed by the oral face (labial mesoderm) and of a portion dependent from the aboral face; this last is more constant, more voluminous, and constitutes the essential portion of the mesoderm; it is derived in most cases from a simple delamination of the exoderm, but in the Entoproctes, the intestine appears also to take part in its formation; it is even possible in Pedicellina that it is derived from a fold at the end of the intestine, and that we may find in the Polyzoa some traces of an enterocole.

From this primitive type, already very complex, the larvæ of the Entoproctes are derived by a differentiation of the mesodermic

1 In all the figures o indicates the mouth; sh and si, the furrow; c, the crown of cilia, mi, aboral mesoderm; c, ciliated crown; est, stomach.

masses, which place themselves in relation with the skin at three different points to form the three tactile organs. The larvæ of the Cyclostomes are formed

by an extension of the crown in the form of a mouth on the aboral face, and finally the larvæ of the Chilostomes (Fig. 4) and Ctenostomes arise by a division of the aboral face into two parts, the cupping glass-like part (ventouse) and lower part, resulting from the withdrawal of this same face.

FIG. 4.-Third derived form of Chilostome larva.

Our author then compares the larval form of Polyzoa with those of the Rotifera, and compares their ciliary crown with the rotary organ of Rotifers, the thoracic segment of Brachiopod larvæ and the ciliated crown of the Trochosphere of molluscs and worms, and find a strong similarity between them. He accepts the Trochosphere-forms of Rotifers, Polyzoa, Brachiopodes, Molluscs

and Annelids, and thinks that there are essentially two forms, the first giving rise to the Polyzoa, the second to the Brachiopods, Molluscs and Annelids; the first group having the ciliary crown placed below the mouth, and the second and last having the crown of cilia placed above the mouth. He is of the opinion that the Polyzoa on embryological grounds are. on the whole, closely related to the Brachiopods and Rotifers, and from

FIG. 7.-Polyzoan larva.

the general re

semblance of the FIG. 8.-Branchiopod larva.

larvæ of the entoproctous Polyzoa to the Rotifers he concludes