Selection undoubtedly expresses a great truth, that a struggle for existence is always going on among the far too numerous offspring of the same parents; and that, where no other causes come into operation, those of the offspring which possess any advantageous differences from the remainder will survive to the prejudice of the rest, and will have a tendency to perpetuate this divergence. When, however, Natural Selection is brought forward as adequate to account for the whole history of biological evolution, it presupposes the principle that no change can take place in the way of the evolution of one species from another that is not directly and immediately to the benefit of that individual species;-in other words, that each form of life exists for its own advantage only. But do we not see around us many facts which appear to negative this hypothesis? Biological forms have been evolved presenting peculiarities of structure, special developments of particular organs, not possessed by their parents, but which, as far as we have any means of judging, are and can be of no special advantage to them in the struggle for life. We seem, indeed, more and more compelled to the conclusion that we know next to nothing of the laws which govern the evolution of species, and the development of the marvellously diverse forms of animal and vegetable life that surround us. I cannot myself get away from the conclusion that we must attribute the tendency to variation which is admitted to be the material on which Natural Selection works, to some inherent force belonging of necessity to the functions of life, whether animal or vegetable, which is independent of, and in some sense superior to, the forces that govern the inorganic world. Above all, we are compelled to recur to the pre-Darwinian doctrine of Design; and to believe that Nature has some general purpose in the different modes in which life is manifested, a purpose not in all cases for the immediate advantage of the individual species, but in furtherance of some design of general harmony which it may take centuries of unwearied observation and laborious toil before we discover the key by which we may be able to unlock it.

11

RECENT MICROSCOPY.

BY HENRY J. SLACK, F.G.S., SEC. R.M.S.

THE

HE object of the following Paper is to select, from records. of the microscopical work of 1871, some matters of the most general interest, giving preference to topics which either suggest inquiries many readers can easily follow, or throw light upon well-known questions that have not hitherto been settled.

First, let us notice various attempts to elucidate the structure of the scales of butterflies and of other insects, which, although not belonging to the Lepidoptera, are furnished with scales of a similar or identical character.

Scales appear on various parts of insects besides their wings, and in many cases it is not difficult to show that the hairs of insects are similar structures to scales, both being modifications of the skin. Insects' wings must not be confounded with those of birds, though both are instruments of flight, acting upon the air in a similar manner. In the case of a bird, it is obvious that when the feathers are removed we come to a structure of bone and muscle, and it is the skin that gives rise to various kinds of feathers, that have a most important part to play in enabling a bird to sustain and guide itself in the air. The wing of the insect is an extension of the skin or integument covering the body, sufficiently hardened to bear atmospheric resistance, and strengthened by nervures that, although possessing none of the structure of bones, act in a mechanical way to give firmness and support. With insects, the scales and hairs are evidently related to similar appendages of higher creatures, but the wing feathers or scales of the insect do not assist flight, like those of the bird. They are an ornamental covering, probably of some use in protecting the membrane from which they spring, and on which we find them arranged like tiles on a roof; but the flying power of an insect does not seem impaired when numbers of them are rubbed off. By their beautiful aspects they make, according to Mr. Wallace's observations, males and females mutually attractive, and they are frequently the means of disguises that enable their possessors

to escape notice of their enemies, as when a leaf-like butterfly perches on a twig, and looks as if it had grown there-one of many surrounding leaves.

These remarks may assist the student in looking for right analogies and avoiding wrong ones in investigating lepidopterous or other scales.

The minute markings of such scales have always been favourite objects for the microscopist to display, and some of them are still regarded as good practical tests for various powers. Among the most difficult to show clearly are certain markings on the scales of insects to which the name of Podura is still popularly given, though entomologists now call them by other appellations. The famous test scales of microscopists come from an insect now named Lepidocyrtus curvicollis; and since Dr. Pigott affirmed that with sufficiently corrected glasses a distinctly beaded structure was to be seen in them, fresh discussion as to their real nature has gone on without ceasing, and strong feelings, as well as reasonings, have been shown by many who had perfectly satisfied themselves with the appearance of the well-known note-of-exclamation marks, so well shown and so beautifully figured by the late Richard Beck. It seems probable that insect scales are essentially composed of two membranes more or less corrugated, forming a sort of quill at the end where their insertion into the membrane takes place, and expanding upwards into a sort of bag, in the socalled "battledore" variety, and into a flattened plume in the ordinary sort. An intermediate membrane has been described by some observers, but this appears only the result of a deposit which in most scales takes a more or less beaded form, and may combine into a distinct layer in some kinds.

Dr. Pigott's" beads are by no means inconsistent with the existence of corrugations, and the exclamation marks are probably true aspects with a particular focussing and illumination, though few observers, who have taken much care in the investigation, have for many years supposed them to afford an accurate and complete idea of structure.

The extreme delicacy of the Podura or Lepidocyrtus scale gives rise to so much difficulty, both of observation and interpretation, that it is advisable to be guided by analogy drawn from easier scales in its interpretation. This plan was pursued by the writer, who traced what seemed to be real beads in ordinary and easy butterfly scales, through more difficult ones, up to those of Lepidocyrtus curvicollis.

Mr. R. J. McIntire took up the question with great skill and with an absence of prejudice somewhat remarkable, in a discussion that has excited an unusual amount of strong feeling; and whatever ultimate conclusion may be reached, his obser

vations and beautiful sketches will have a permanent value. Taking some scales from the jumping spider(Scenicus salticus), and carefully viewing them with high powers, he found their edges "crenated," the "outer membrane" smooth, and the "inner membrane," or that next the creature's body, "puckered up into somewhat irregular rows of hackles." This seemed like a hint that the interjection markings of the Padura might be due to corrugation. In scales of Polyxenus lagurus he found, what was "very uncommon," according to his observations, "a deposit between the membrane," and the scale was a very solid structure. Most of his endeavours to detect beaded deposits led him to think such appearances were only "ghosts;" and it is well known that false appearances of beading are easily produced under certain conditions. Mr. McIntire's account of his observations and experiments scarcely warrants his conclusions, for he admits "pigment granules" in scales such as Amalthusia Horsfieldii, figured long since by Mr. De la Rue, and in some others.

Lieut.-Colonel Dr. J. J. Woodward, of the U.S. army, employed his well-known skill in photographing Podura and other scales. In the April number for 1871 of the "Monthly Microscopical Journal" will be found a paper by him, read before the Royal Microscopical Society, in which he says: "On the coarser De-. geeria scale (D. domestica), I had no difficulty in making out appearances which, so far as I can gather from Dr. Pigott's own descriptions and the published discussions of his views, are substantially the same as those seen and shown by him. . and even on the more minutely marked and difficult Lepidocyrtus scale I have been able to develop appearances which seem to be substantially similar." Dr. Woodward did not, however, pronounce any decided opinion as to real structure; but since the date of this paper he has kindly forwarded to the Royal Microscopical Society, and to the writer, photographs of Degeeria domestica beautifully exhibiting a beaded appearance. In a communication to the Royal Mocroscopical Society, read in May 1871, Dr. Woodward speaks of Mr. Joseph Beck having shown and left with him a fine Podura slide, showing the note-of-exclamation marks with remarkable clearness; " but immediately afterwards, with the same optical combination and magnifying power, without any change in the cover correction, by simply rendering the illuminating pencil oblique, and slightly withdrawing the objective from its first focal position, he obtained a negative which displayed the bead-like or varicose appearance of the ribbing more satisfactorily than he had previously been able to do." A photograph of this appearance may be seen at the Royal Microscopical Society's room.

See "Monthly Microscopical Journal," January 1, 1871, &c.

6

Dr. Maddox took up this much controverted scale question, and sent a paper to the last-named society in May 1871, accompanied with a series of careful drawings given in the Monthly Journal' for June. This paper should be referred to, and the drawings carefully examined, in order to appreciate his work. He took a great deal of pains with chemical solvents to remove oily matters, and finally made out a ribbed structure, to which he thought the beaded aspects were due, as fine ribs crossing each other would give that effect. It does not, however, seem that the existence of such structures as Dr. Maddox figures negatives the existence of deposits in a more or less beaded form, nor do the investigations of Mr. Wenham, which prove the reality of surface irregularities more or less corresponding with the exclamation marks.

The reader may by this time have had enough of butterfly scales, and we turn to another subject, also entomological-a demonstration by Mr. Lowne that the so-called "suckers" on the feet of the water-beetle (Dytiscus marginalis) are not suckers at all, but an apparatus resembling the cushions attached to the feet of blow-flies for exuding a sticky fluid by which the creatures can be sustained in opposition to the force of gravity. The cushions, or pulvilli, of the Dytiscus and other insects seem to have been taken for suckers without any sufficient ground for such an opinion, and when it seemed a probable guess that they and other insects capable of strongly attaching themselves by their feet did so by some sort of airpump mechanism.

Mr. Lowne found the pulvillus of the Dytiscus a mere modification of the structure of that organ as seen in common flies. He traced in it a secreting sac, supplying a viscous fluid, which percolated through "disk-bearing hairs.' He showed that a dytiscus, made insensible by chloroform, mechanically adhered to the inner glass of the receiver of an air-pump when the air was exhausted, and when, if atmospheric pressure had caused the adhesion, it must have fallen. Common flies are frequently found dead and adhering to window panes, the sticky fluid having hardened while they were alive and rendered them prisoners after their decease. Mr. Lowne has noticed in some dytisci a loss of tarsal disks, apparently from their having been allowed to adhere too tight, so that the insects had to pull their legs away without them.

During the past year numerous papers have appeared relating to the various forms of minute life, somewhat jumbled together under the now popular name Infusoria, and, as usual, the spontaneous generation controversy has continued without exhibiting any symptoms of final settlement. Indeed, it seems more likely to be decided at last by reasoning from a large