rays." True, but the ordinary mode of conceiving the process we have just seen to be untenable. When Von Siebold says that the "mysterious phenomenon rests only on an imperfect knowledge of the structure of the organ," he seems to me to forget that the phenomenon is by no means peculiar to the Cephalopoda, but is characteristic of the Invertebrata generally. What, for instance, is the simplest form of an eye, disregarding those hypothetical "eye-specs" which have been noticed in Infusoria? It is that of a pigment spot on a ganglion, or a nervous expansion. Ascending higher in the scale, and reaching even the complex structure of the crab's eye, what do we find but a pigment layer covering the retina? If certain processes do pass through the pigment from the retina, it is very questionable whether these are nervous in structure, and, if nervous, they are still only conducting-threads insensible to the direct influence of light. They are held to be analogous to the rods and cones of the Vertebrata, which, as we have seen, receive their stimulus from changes in the pigment, not directly from the light. It is thus, as Leydig says, "in the Vertebrata the rods form the outermost layer of the retina; in the Invertebrata they form the innermost. Herewith is connected the fact, which at first seems so surprising, that the choroidal pigment lies in front of the retina, therefore the contrary of what occurs in Vertebrata." In the blind Crustacea no pigment is present; and in Albinos, in whom the pigment is of lighter colour, vision is imperfect. If we remember that, according to the hypothesis, light only affects the retina after changing the temperature of the pigment, which change is communicated to the rods and cones, and

thence to the vesicular layer, there will be nothing paradoxical in this inverse arrangement of the retina in Invertebrata; in both the process is essentially the same, and the mere difference of position is not more than the difference of the chain of ganglia, which in the Vertebrata is dorsal, and in the Invertebrata ventral§

Returning from this digression, and its surprises, to the eyes of our Nudibranchs, we can have little doubt that their vision is simply the perception of light and darkness. The changes of temperature produced by the alsorption of the rays in their pigment cannot be elevated into the percep tion of an image, because the optical conditions for the formation of an image are absent: an indefinite sensation, resulting from change of temperature, is all that they can perceive, Nay, even were their eyes constructed so as to form optical images, there is little doubt that vision, in our human sense, would still fail them, owing to the absence of the necessary combination of tactile sensations with sensations of light. We see very much by the aid of our fingers.

Apropos of tactile sensations, are those anatomists who assume the existence of invisible nerves in parts of the skin which, although revealing no nerve to the eye, seem to reveal it to the mind by the manifestation of sensibility, warranted in such an assumption? Kölliker has shown that there is no portion of the skin, however minute, which is not sensitive. But does this prove that every point must be supplied with a nerve? Admitting that sensibility resides only in nerve-tissue (which for my part I doubt, and next month will furnish my reasons), I think another explanation will do away with such an assumption.

LEYDIG: Histologie, p. 253.

It is unnecessary that a nerve-fibre should be directly pressed upon at the immediate point of contact of the needle and the skin. The sensation will equally result if the pressure be communicated at some distance from the point of contact. Strictly speaking, this is always the case when the cuticle is not pierced. The needle presses on the cuticle, and the pressure is communicated from the cuticle to the nerve; and it is evident that this pressure may be lateral as well as perpendicular. If a nerve be within the range of this lateral pressure, it will be affected; and although those parts which are liberally supplied with nerves are necessarily more sensitive than others, because more filaments come within the range of lateral pressure, yet no part of the skin is insensible, because no part is without the range of a

nerve.

Having proved that our Molluscs cannot see, we have now to inquire whether they can hear. As in the former case, the answer must depend on what is meant by "hearing.' If every sensation of light and darkness is to be called sight, and every sensation of sound is to be called hearing, our friends certainly both see and hear --as blind men see, and deaf men hear. Let us examine the organ in a Doris or Pleurobranchus instead of the complex structure found in higher animals, we find a microscopic vesicle containing pebbles suspended in liquid. In the Doris this vesicle has no nerve, but lies upon the cerebroid ganglion, immediately behind the optic ganglion. Nor have I, in a dozen dissections, been able to detect a nerve in the Pleurobranchus, although Krohn describes one in the sub-genus, Pleurobranchæa. At any rate, embryology proves the nerve to be a subsequent addition, since in the embryos of all the Nudibranchs the ear is a simple vesicle containing a single otolithe, with neither nerve nor ganglionic attachment. The mention of embryological indications reminds me that Von Siebold has

shown the close analogy which exists between the permanent organ of hearing in the gasteropod molluscs, and the transitory form of that organ in the embryo of the fish.

With such an organ, a mere bag of pebbles in liquid, what degree of that exquisite sense, known by us as Hearing, can be claimed by the interesting animal which naturalists are fond of styling "the humble mollusc?" I never detected any humility in my molluscs; and if they seem humble in the eyes of haughty ignorance, a little knowledge of their structure will soon remove that misconception. It is true, they give no dinners, and are perfectly regardless of the higher circles; they trouble themselves very little about any of the " great movements;" they do nothing for the "Progress of the Species;" leave the Jews unconverted; have no views on the "Ballot ;" and are utterly insensible to the advantages of " Marriage with a Deceased Wife's Sister.' But they have their little world, and are as perfectly constructed for it as we are, who condescend to notice and patronisingly admire them.* In that world they do not need what we need. They hear nothing of the marvellous inflections of speech, the tremulous tenderness of affection, the harsh trumpet - tones of strife, the musical intonations of mirth. They cannot hear the prattle of children's voices, which send such thrills along our nerves; nor can they hear the untiring eloquence of a vexed virago, which also sends thrills not of so pleasant a nature. Deafer than the deafest adder will they remain, charm we never so wisely. Equally insensible must they be to music. Beethoven's melodious thunder, Handel's choral might, Mozart's tender grace, Bellini's langorous sweetness, are even more lost on them than on the lymphatic dowagers in the grand tier, who chatter audibly of guipure and the last drawing-room, while Grisi's impassioned expression, and Mario's cantabile are entrancing the rest of

the audience. The Mollusc can only perceive noises. Sounds are by us separately recognisable in their intensity, their pitch (or note), and their quality. The Molluse only recognises intensity-loudness. A wave of sound agitates the otolithes in his ear, and their agitation communicates to the ganglion a sensation of sound, loud in proportion to the agitation.

Had we no other evidence, this would suffice to show the error of the

vulgar conception of hearing. Sound is not produced by waves of air striking the drum, these waves being thence transmitted along the auditory nerve to the brain; but the waves agitate the sensory apparatus, which in its turn acts upon the Sensational Centre. That is why sounds are heard with painful distinctness when the sensory apparatus is affected by other stimuli besides the pulsating of waves of air. Few subjects are of greater interest to the philosophic mind than the gradual complication of the organ of hearing, with, of course, its proportional complication of function, in the animal series. Even in human beings we see differences only less considerable than those which exist between man and animals. The ear of one man is utterly incapable of appreciating those delicate intervals and harmonic combinations which give to another exquisite delight. The bird,

"Singing of summer in full-throated ease,"

is insensible to music, and probably distinguishes nothing in speech except the loudness of the tone. And this fact may lead us to question whether the general notion, so often insisted on, of the superiority in the senses of animals over those of man is not a fallacy. It is quite true that a bird sees distinctly at greater distances than a man; but can it see such delicate nuances of colour? A dog perceives some odours to which we are insensible; but in the immense variety of odours we are capable of perceiving, our superiority is manifest. In hear ing, animals are demonstrably inferior. Some of them may be as susceptible to certain sounds, but none are susceptible to the immense variety of sounds distinguishable by

our ears. Indeed, when from Comparative Anatomy we learn how much more complex are the Sensational Centres in man, we may be certain that the sensations will be more various.

Before quitting our Molluscs, let us for a moment consider the shells with which the vast majority are furnished, and with which all are furnished in their embryonic stage. I do not mean that we should lose ourselves in the varieties of a concholo

gist's collection, nor that we should inquire minutely into the structure of the shell and its mode of growth; but that we should pause to consider its relation to the great forces of the universe. You may possibly look upon that phrase as inere rhetoric; but it is of strictly scientific sobriety; and you will admit it to be so, on learning that the mighty ocean-currents mainly depend on this said mollusc-shell. Strange, yet true. Were there no secreting animals in the sea capable of removing from the water its surplus lime, the stormy winds might agitate its surface, and rouse its waves like troops of roaring lions shaking back their manes of spray; but there would be no strong currents with beneficent effect; and in a little while the ocean would become a huge salt lake. Let us rest from our hot hammering and painful stooping under ledges, and let us enjoy a few minutes' repose on this reef, solitary amid the waves, and distant from the shore. Pleasant the breeze, pleasant the gentle cadence of the water at our feet, pleasant the sight of that snowy mass of cloud which lazily rolls landwards. It rose from the surface of this brilliant, buoyant, volitant sea in airy bubbles of vapour, and is now travelling towards those green cornfields over which the lark is poised in melody. If the cloud should there meet a current of cold air, it will drop gently down as rain. This rain will make its way through the earth to rivulets and rivers, till it finally returns once more to the parent-bed of ocean; but on its way it will have washed with it various salts, which it will dissolve and carry to the sea, thus adding to the already saturated sea-water an amount of solid matter such as

would impede its flow, were there no provision ready to restore the equilibrium. For observe, the rain-cloud, as it rose by evaporation from the sea, left behind it all the salts which it contained, and these would make the rest of the water denser; but now the rain-cloud returns laden with as much salt as it originally had, and the very fluidity of the sea is in peril, since evaporation is incessantly going on, and rivers are incessantly returning laden with lime. What becomes of this excess of lime? Polypes and Molluscs, Crustacea and Fish, but mainly the two former, clutch hold of it, wring it from the water, and mould it into habitations for themselves. It is thus vast coral islands and oyster-beds are formed. The sea is a great lime-quarry; but the lime is arranged in beautiful forms, and subserves a great organic end. Not only are animals thus furnished with houses and solid structures, but the water, thus relieved of its excess, is enabled to flow in mighty currents. This is the theory propounded by Lieutenant Maury in his fascinating book.* Assuming the

waters of the sea to be in a state of perfect equilibrium, the animals would, by their secretion of salts from it, produce currents :

"The Mollusc abstracting the solid matters has by that act destroyed the equilibrium of the whole ocean, for the specific gravity of that portion of water from which this solid matter has been abstracted is altered. Having lost a portion of its solid contents, it has become specifically lighter than it was before; it must, therefore, give place to the pressure which the heavier water exerts to push it aside, and occupy its place; and it must consequently travel

about, and mingle with the waters of the other parts of the ocean, until its proportion of solid matter is returned to it. The sea-breeze plays upon the surface; it converts only fresh water into vapour, and leaves the solid matter behind. The surface thus becomes specifically heavier, and sinks. On the other hand, the little marine architect below, as he works upon his coral edifice at the bottom, abstracts from the water there a portion of its solid contents; it therefore becomes specifically lighter, and up it goes, ascending to the top with increased velocity to take the place of the descending column, which, by the action of the winds, has been sent down loaded with fresh food and materials for the busy little mason in the depths below."

Was I not justified in saying that the Mollusc was deeply interesting in its relations to the great forces of the universe? Does not this one example show how the great Whole is indissolubly connected with its minutest parts? The simple germination of a lichen is, if we apprehend it rightly, directly linked with the grandest astronomical phenomena; nor could even an infusory animalcule be annihilated without altering the equilibrium of the uni

How many Englishmen would visit Madrid within a year after the opening of a railroad between that capital and Bayonne? We have heard a very competent judge of English roving propensities and love of new tracks, fix the probable total at ten thousand. Allowing for round numbers and a rough estimate, this perhaps would prove not much beyond the mark. For what European capital is there, half so accessible as Madrid then would be, that is known to so few Englishmen? The railway made, you steam in fifty hours from London Bridge to the Puerta del Sol. Twenty-five years ago, you were nearly as long getting to Paris. What now deters foreigners from visiting Madrid is apprehension of the latter half of the journey; of bad roads, uneasy vehicles, filthy inns, odious insects, and an option between starvation and viands floating in oil and fragrant with garlic. Some of these inconveniences must be encountered, but others are exaggerated, and the distance itself is inconsiderable. Guidebooks make it somewhat less than one hundred leagues; but the leagues are short for Spain; and, notwithstanding stoppages for food, great loss of time in changing horses, and three lines of mountains to crawl over, the mail does it in forty-eight hours when the weather is fine. In winter, when the snow-drifts are deep on the lofty ridges and passes of Salinas, Descarga, and Somosierra, the time of travel is very uncertain-often between sixty and seventy hours. As regards distances, however, in Spain it is difficult to ascertain them exactly, there being nothing exact in Spain, just as nobody is punctual. Exactness and punctuality are for busy and industrious countries-not for this one, where few persons have anything to do, few do it who have, and time is valueless. The English and French talk of killing time, but the Spaniard has a different name for idleness. When you find him, as you may any day and all day, rolling his paper cigar and gazing vacantly on vacancy, he tells you he is mak

ing time-hacer tiempo. To return, however, to the Madrid road. It is not much better than its reputationat least after you leave the Basque provinces behind you-and we may doubt whether tourists will venture upon it in considerable numbers until such time as the rail be installed. There is no saying how long that will take; for, to say nothing of natural difficulties, such works go slowly on in Spain. The rail completed, however, Madrid will doubtless, for a time, be much resorted to by English. And, to speak without prejudice, there are capitals less worth seeing. Lovers of art would think little of a fifty hours' pilgrimage, when at its termination their eyes were to be regaled by the wealth and gems enshrined in the Madrid picture-gallery, including nearly all the works of Velasquez, that prince of Spanish painters, including, too, a prodigal collection of the pictures of Murillo, Raphael, Rubens, Titian, Teniers, and of a host of other names, of all schools and nations, whose sound alone is fame. Although some of the great Italian artists are represented scantily or not at all, the collection is generally admitted to be the finest and most copious in the world. To see it properly is the work, not of days, but of weeks, and for a fanatico per la pittura there are months of enjoyment in its spacious galleries and well-filled saloons. The eye is at first bewildered by the riches that on all sides present themselves, and it is not until after two or three visits spent in desultory wandering and wondering, that one settles down to a methodical examination. Few galleries can be seen with greater comfort and satisfaction, for the lights are generally good and the visitors not numer

ous.

Modern Spaniards are by no means enthusiasts for art; and a few artists making copies, and a stray foreigner or two, are generally all that one encounters there. For the foreigner the Musco is the great marvel and attraction of Madrid. The next thing worth seeing here is a bull-fight, a national sport to which