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brown. The fracture is conchoidal, without any trace of cleavage. The surface of the crystal is shining and smooth, but the fracture surface splendent; and lustre between vitreous and resinous. Its constituent parts are as follows: Titanic acid 62.75, lime 12.85, oxide of uranium 5.18, oxide of cerium 6.80, oxide of manganese 2.75, oxide of iron 2.16, oxide of zinc 0.61, water 4.20, fluoric acid, quantity not determined, magnesia a trace, 97.30.

The Law of the Preservation of Species, illustrated by the Phenomena of the Seed of the Stipa pennata. By Mr JOHN MACVICAR, Lecturer on Natural History in St Andrew's. (With a Plate.) Communicated by the Author.

ALTHOUGH it cannot be said that the primary object of nature, in reference to a species, is to prevent its destruction, yet its existence is an essential condition to that end, whatever it may be, and accordingly, nowhere do we observe a more admirable mechanism, than in those organs which are most eminently conservative or reproductive. The general law by which their developement and efficiency are regulated, may perhaps be thus stated, that, in proportion as the causes operating to destroy a species increase, so also do the organs or functions operating to preserve it.

Thus, as we descend the scale of animated beings, the successive species become more and more restricted in their faculties, their cunning, or swiftness, or force, by which they may meet their enemies, the number of which is also increased, or in those resources by which they may survive the violent action of the elements, which beat upon their more minute and simple structures, as rudely, and as boisterously, as upon the more perfect animals. Their liability to destruction, then, becomes greater as we descend. But to counterbalance this, we find that, in obedience to the law which has been stated, the very degradation of their structures becomes subservient to their existence. For, by a collateral diminution of sympathies, the life of the individual becomes more independent of partial in

juries, and a tenacity is imparted to it, which would even be ridiculous in the higher animals. Thus, it is very absurd to think of a man continuing to live after his head had been cut off; yet low in the scale, we find many species which, when decapitated, can serve themselves with new heads, as efficient as those of which they had been deprived, and scarcely differing from them, but in their paler complexion. Of this circumstance Mr Dalyel availed himself, in his very interesting investigation of the Planariæ. For when he wished to know how many eyes the Planaria nigra possessed, not being able to distinguish them on account of the black colour of the animal, he decapitated several, and was then able to count the eyes in the pale reproduced heads. As to legs, the amputation of one of which without surgical aid, would prove inevitably fatal to a man, there are many animals which seem to part with them without much inconvenience; while there are others (as the crabs), which, according to recent observations, seem to scorn the possession of a leg when injured, casting it triumphantly from them.

If we descend still farther among animals still more beset by enemies and accidents, we find species which really seem to be "immortal under the edge of the knife," which to cut in pieces, is only to give being to so many individuals as perfect as that which was attacked.

The action of the same admirable law is illustrated in the reproduction of the race. Thus in the most perfect animals, the species is divided into two groups, only one of which is capable of producing offspring. As we descend, this bisexual character is obliterated, and every individual, often without the presence of another, acquires this power. Still lower, not only do we find each animal provided with a specific apparatus for this purpose, but the same end accomplished in other ways also, as by gems and spontaneous division.

parallel to that of aniThus the oak, which

In the vegetable economy, which runs mals, we observe the same law to operate. cannot easily be destroyed, the individual life of which survives

the sweep of many ages, can only be reared from an acorn;

while the tender moss, which springs up among the turf beneath which its roots are spread, or the parasitic lichen on its trunk and branches, the lives of which are subject to a multitude of

accidents; may be propagated both by sporules produced in proper seedvessels, by germs and otherwise.

But besides this beautiful law, the action of which may be distinctly recognised, preserving the species of organised beings in existence, notwithstanding the perpetual destruction which they wage against each other; we are able to observe the traces of another no less beautiful, that, in proportion as a species is useful in the economy of nature, so are the developement and ef ficiency of the organs and functions that effect its diffusion.

This might be inferred a priori, from what we know of the attributes of the Creator, and the analogy of his works. This, however, is a mode of reasoning not admitted in Natural History, in which a law must only be framed, as a generalised statement of a number of observed phenomena, tending to a common purpose. But that such a law exists we observe many traces of evidence. Thus there is no tribe of plants more eminently useful in the economy of nature than the grasses, the foliage and seeds of which supply the first necessaries of life, not only to man but to a multitude of the inferior animals. And, perhaps, in no tribe equally highly organised, do we observe the same tenacity of life, or the same economy and care in the reproductive organs, to avoid the introduction of parts that might be easily injured, and so prevent a successful fructification.

In the grasses, the delicate coloured flower that gives so much beauty to most other tribes, is replaced by concave husks, which are not only most hardy, but so situated that the weather can scarcely penetrate to injure the essential organs within. Besides this, the peculiar structure of the embryo, which admits of a number of stems from one seed, might be mentioned, the copious albumen, &c. But I proceed to describe, and a few words will suffice, the beautiful structure of the awn exhibited in a species of this family, which effects the introduction of the seed into the soil so wonderfully, that I cannot satisfy myself with admiration.

The Stipa pennata is a most elegant species of grass, which, though not a native of Scotland, thrives luxuriantly in the open border. Its seed is closely invested by the glumaceous perianth, which consists of two husks, a larger and a smaller, the former of which overlaps ofdges of the latter, and almost entirely

envelopes it. Thus the strong outer covering of the seed is produced below into a very sharp rigid spine; and terminated above by a long awn, which is articulated to its summit. Originating near the base, and proceeding up certain ridges on this the investing valve chiefly, are lines of stiff hairs pointing upwards. The awn, when fully developed, is about thirty times the length of the seed, or about fourteen inches. It is round, tapering and plumose, with the exception of about three inches at the base, which are compressed, longitudinally sulcated, and without hairs.

The seed, therefore, and its appendages, possess a structure such as is imitated in a barbed and feathered arrow, which is so well calculated to find its way into the ground in a vertical direction. Many seeds, however, possess a similar structure, and it is not this which gives to the awn of the stipa its most striking peculiarity. It is a change which takes place upon the awn, after it has left the plant that produced it. When it has fallen from the parent plant, it enters the soil vertically, and in a few hours the base and sulcated part of the awn becomes twisted, and the feathered portion becomes horizontal. In consequence of which, it is blown round by the autumn winds like a vane, and every turn screws it farther down into the earth; for the hollows and ridges which, when it remained upon the plant, were only longitudinal sulci, have now given rise to the hollows and elevations, in a word, to the threads of a screw. Thus it is moved down, and whatever is gained, is prevented from being undone by a reverse motion of the vane, in consequence of the stiff hairs upon the glume which act as barbs.

When it has been thus worked down into the moist soil, into the situation most favourable for germinating, the attachment between the awn and seed is dissolved; for having drawn up many when they were in this condition, I have invariably procured the awn only, and never, by any chance, the seed. Such appears to be the function of the "spiral articulated deciduous awn" of this interesting species *.

The seeds of the Stipe often occasion great inconvenience and trouble to travellers, and even to the domestic cattle of the districts where they grow. This fact is well stated in the following notice by Mr Raspail.-ED.

"On the morbid accidents to which animals are exposed by the seeds of Stipa pennata and capillata.It is known that the husks. genus Stipa terminate

The accompanying drawing represents the seed and its appendages, more or less magnified.

Explanation of the Figures in Plate I.

Fig. 6. The two valves of the glumaceous perianth, with the stiff hairs, the spine and articulated awn.

7. The grain, with part of the skin torn at the base, to shew the albumen, of which nearly the whole is composed, the cotyledon and the embryo.

8. The seed, with a fourth part of the awn, to shew its form when ready to separate from the spike.

9. The same, as it appears some hours after separation.

Account of the Observations and Experiments made on the Diurnal Variation and Intensity of the Magnetic Needle, by Captain Parry, Lieutenant Foster, and Lieutenant Ross, in Captain Parry's Third Voyage, with Remarks and Illustrations. By PETer Barlow, F. R. S. Mem. of the Imperial Academy of St Petersburgh, &c. (With a Plate.) Communicated by the Author.

As the experiments referred to in the head of this article were performed under such extraordinary advantages of locality, of

at the base in a reversed cone, which is very sharp, and covered with stiff hairs directed upwards, so that when the point penetrates into any substance, the hairs not only prevent it from coming out, but contribute to make it go deeper. M. Desfontaines, in his Flora Atlantica, and M. Lamarck in the Encyclopedie, have pointed out the inconveniences to which a seed so organised subjects travellers passing over the fields of Barbary, Greece, and Portugal, at the time of ripening of the stipas. The seed penetrates into their clothes, and sooner or latter disconveniences them in a high degree, by producing scratches of various depths upon the skin. A great mortality of the cattle, which took place in 1823, in the neighbourhood of the village of Berczel in Hungary, afforded an opportunity to the Professors of the Royal University of Pesth, of making known a still more singular effect produced by these seeds. It was found that the seeds of the stipas, which abound in the pasture grounds of Berczel, stuck to the wool of the sheep, penetrated into the skin, and even made their way to the internal organs. On dissecting a great number of these sheep, seeds were found in the vicinity of the liver and in the peritoneum, and the skinyamined between the eye and the light, had the appearance of Lave alread. As these grasses occur in all the southof Dece

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