In Chloëon, though the changes are gradual, the difference between the larva and the imago is very considerable, and we have seen that the action of external forces produces changes which have no reference to the form of the perfect insect. In caterpillars we have a typical class of metamorphoses. Until recently, however, we knew of no case in which a larva produced more than one perfect insect.* Insects never multiply by buds, and almost always the external form is acquired before the organs of reproduction are mature. Recently, however, Professor Wagner of Kasan has discovered that the larvae of certain Cecidomyias have the faculty of producing other larvæ, so that they present a true case of alternation of generations. Thus, then, we see that insects present every gradation from growth to alternation of generations; we see how, from a single fact, metamorphoses and alternate generations may have originated, and we find reason to suppose that in the course of time the latter phenomenon may become more frequent than it is at present. It is, moreover, evident that there are in the animal and vegetable kingdoms two kinds of Dimorphism. The term has generally been applied to those cases in which-as in the ant and bee, in animals, and the Primulas among plants-the perfect individuals are divided into two forms. In fact, the sexes themselves constitute a kind of Dimorphism. In these cases the forms are not alternate. When, however, external forces act on the young in one manner, and on the mature form in another, they tend to produce different forms, which do not complement, but succeed, one another. I have elsewhere proposed to distinguish this form of dimorphism, under the name of Dieidism or Polyeidism. In Polymorphism the chain of being divides at the extremity; in Polyeidism it consists of dissimilar links. Finally, the speaker said :-"The principal conclusions which I would impress on you this evening are: 66 "1. That the presence of metamorphoses in insects depends, in great measure, at least, upon the early state in which they quit the egg. "2. That metamorphoses are of two kinds-developmental and adaptational. "3. That the apparent abruptness of the changes which they undergo arises in great measure from the hardness of their skin, which permits no gradual alteration of form, and which is itself rendered necessary in order to afford sufficient support to the muscles. "4. That the immobility of the pupa or chrysalis depends on the rapidity of the changes going on in it. "5. That although the majority of insects go through three wellmarked stages after leaving the egg, still a large number arrive at maturity through a somewhat indefinite number of slight changes. *The instances in which certain insects breed while their wings are but imperfect, might here have been cited. But as there is much difference of opinion among entomologists as to these cases, I have thought it better to take one about which no question is likely to arise. "6. That the form of the larva of each species depends in great measure on the conditions in which it lives. "When an animal is hatched from the egg in an immature form, the external forces acting upon it are different from those which affect the mature form, and thus changes are produced in the young, bearing reference to its present wants rather than to its ultimate form. 7. When the external organs arrive at this final form before the organs of reproduction are matured, these changes are known as metamorphoses; when, on the contrary, the organs of reproduction are functionally perfect before the external organs, or when the creature has the power of budding, then the phenomenon is known as alternation of generations. "Insects present every gradation, from simple growth to alternation of generations. 8. Thus, then, it appears probable that this remarkable phenomenon may have arisen from the simple circumstance that certain animals leave the egg at a very early stage of development: and that the external forces acting on the young are different from those which affect the mature form. 9. The dimorphism thus produced differs in many important respects from the dimorphism of the mature form which we find, for instance, in the ants and bees; and it would therefore be convenient to distinguish it by a different name. "But there is still another aspect under which, if time had permitted, the metamorphoses of insects might have been regarded. In one or two cases, indeed, I have sketched very briefly and imperfectly the habits and mode of life of particular insects. A whole course of lectures might be filled with such life histories. The various manners in which different insects provide for the wants of their young are most remarkable; and all the more so because these wants are so different from those of the perfect insects themselves. "Thus, the butterfly which lives on honey, and did live on leaves, lays her eggs on a twig. She seems to feel that honey will not suit her young, and that the leaves will wither and fall before another spring comes round. "The gnat, which lives in the air and feeds on blood, lays her eggs on the surface of water; and the sugar-loving housefly knows that very different food is necessary for her young. "The nut-weevil chooses the embryo of the nut; the goat-moth the bark of the willow; the Rhipiphora braves the dangers of the wasp's nest; the (Estrus lays on cattle; the Ichneumon in caterpillars; the gall-fly in the still almost imperceptible bud; and some insects even in the eggs of others. "Generally, the larvæ forage for themselves; but, in some cases, the mother supplies her young with food. Thus, the solitary wasp builds a cell and fills it with other insects. If, however, she imprisoned them while alive, their struggles would infallibly destroy her egg; if she killed them, they would soon decay, and the young larva, when hatched, would find, instead of a store of wholesome food, a mere mass of corruption. To avoid these two evils, the wasp stings her victim in such a manner as to pierce the centre of the nervous system, and the poison has the quality of paralyzing the victim without killing it. Thus deprived of all power of movement, but still alive, it remains some weeks motionless and yet fresh. 66 But, perhaps the ants are the most remarkable of all. They tend their young, they build houses, they make wars, they keep slaves, they have domestic animals, and it is even said that in some cases they cultivate the ground. "Nor must it be supposed that even now the habits of insects are anything like thoroughly known to us. In spite of Reaumur and De Geer, the two Hubers, and many other excellent observers, there is in this subject still a wide field for patient and conscientious labour; the observations already made have been far from exhausting the mine, though amply sufficient to prove the richness of the ore." [J. L.] WEEKLY EVENING MEETING, Friday, March 16, 1866. SIR HENRY HOLLAND, BART. M.D. D.C.L. F.R.S. President, in the Chair. BALFOUR STEWART, Esq. F.R.S. On the Existence of a Material Medium pervading Space. THE question, whether interstellar and interplanetary space is a plenum or a vacuum, has for a long time engaged the attention of the scientific world. As we can hold no communication with these distant regions, except through the light which reaches us, it is to this agent we must look to enable us to answer this question directly or indirectly, either from its own properties and nature, or from its faculty of revealing to us the position and motion of the heavenly bodies, or from both of these together. This twofold aspect of the problem gives rise to the following questions : Question first.-Does the nature of light and radiant heat induce us to believe that space is a plenum or a vacuum? Question second.-Is there anything in the motions of the heavenly bodies that gives us any information on this point? These two questions comprise the observational evidence on the subject. By observational, as distinguished from experimental, we mean evidence derived from a region where we may observe what is going on, but into which we cannot transport ourselves so as to make any experiments; besides this evidence we may moreover make experiments on the surface of the earth in our laboratories, and derive from these experiments a certain amount of information bearing upon our question. We have thus altogether three sources of evidence. First.-Evidence derived from the nature of light and heat. Possibly also the force of gravitation which is exercised by bodies at a distance, and the connection between solar spots and terrestrial magnetism, discovered by General Sabine, imply the existence of an interplanetary medium; but it is better to confine ourselves to light and heat, which differ from other influences in this respect. We know that light and heat travel with a certain velocity, and we can suppose, as it were, a slice of light to be cut off half-way between the sun and the earth; here then we have a certain amount of energy neither in the sun nor in the earth, but half-way between; we cannot, however, at present, make any such assertion with regard to gravity or magnetic action. To begin with the evidence derived from the nature of light and heat. It is supposed by some (or rather perhaps it has been supposed, for the advocates of this theory are dying out) that light consists of exceedingly small particles which are projected into space by a luminous body on all sides, particles having different properties, but all of which, nevertheless, pass through interplanetary space with the same enormous velocity of 190,000 miles per second. On the other hand it is supposed that light consists in the transmission of some sort of motion of a medium pervading all space. The difference between these two hypotheses may be explained in a very few words. The theory of emission supposes the transit of an individual particle from the luminous body to the eye; the theory of undulation or similar theory, on the other hand, merely supposes the progressive motion of a state of displacement between the luminous body and the eye. (Illustrated by an experiment.) Thus, according to the former theory, each ray of light which passes between the sun and the earth is equivalent to the bodily transmission of a set of particles 90,000,000 of miles; while, according to the theory of undulation, the greatest amount of relative displacement of the ether whose vibrations form this ray, is probably much less than the millionth of an inch. Now, the difference between these two modes of viewing light as concerns our subject is this: The existence of an ethereal medium does not appear to be inseparably connected with the first theory or that of emission. The two ideas may be held and have been held together, but they do not appear to be inseparably connected; on the other hand, if light and heat consist of the transmission of some sort of motion of a medium pervading space, we have to start at once with the hypothesis of such a medium. Hooke and Huyghens were the scientific authors of the undulatory theory of light, while Newton, on the other hand, was the great advocate of the theory of emission. The scientific repute of Newton seems to have retarded the progress of the undulatory theory for nearly a century; but of late years it has been revived and extended in this country by Young, whose name is inseparably connected with the Royal Institution, and in France by the illustrious Fresnel. It is only by an appeal to experiment that we can decide between the claims of these two rival theories, and we have to ask ourselves which best accords with the phenomena of optics. First of all, if we assume the hypothesis of emission, it is not easy to conceive why the luminiferous particles discharged by so many different kinds of bodies should all pass through space with precisely the same velocity. In order to escape from this difficulty, it was suggested by Arago that particles may be originally projected with different velocities, but that there is only one of these which is adapted to our organs of vision. This, however, is a very lame explanation, and the necessity for such explanations is one of the characteristics of a bad hypothesis. For if a theory be good, it is wonderful how many facts it will account for without any additional assumption, but a bad hypothesis requires to start an additional assumption for almost every new fact, until at length some one arises which cannot be won over by even this method, and so the hypothesis fails. In the next place, it is difficult to conceive why particles, even although exceedingly small, moving with such enormous velocity, should not inflict on us terrific blows on account of their momentum ; and it has been calculated that if the weight of a molecule of light amounted to but one grain, its momentum would equal that of a cannon ball 150 pounds in weight moving with a velocity of 1000 feet per second. Even although a molecule of light should be many million times smaller than this, its momentum would still be sensible; and as millions of millions must enter the eye every instant from every visible point of every visible body, we should be pounded to atoms. A good many years ago, Mr. Bennet made some experiments on this point. A slender straw was suspended horizontally by means of a spider's thread, and to one end of this lever a small piece of white paper was attached, and the whole was enclosed in an exhausted receiver of glass, while the sun's rays, concentrated by a large lens, |