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56 I had kept these tritons four years in the hope that they would breed; but in spite of their being subjected to great varieties of treatment—for months well supplied with food, and for months reduced almost to starvation—they never showed the slightest tendency to breed; another among the many illustrations of the readiness with which the generative system is affected even in very hardy and not very impressionable animals. Claparède observed the still more surprising fact that the Neritina fluviatilis (a river snail) not only will not lay eggs, but will not even feed in captivity. He attributes it to the stillness of the water in the aquarium, so unlike that of the running streams in which the mollusc lives. See Müller’s Archiv, 1857.
57 Bronn, Morphologische Studien über die Gestaltungs-Gesetze, 1858. Compare the note on § 11.
58 Darwin, On Domestication, II. 340. In the Annales des Sciences, 1862, p. 358, M. Malm describes a fish in his collection, the tail of which had been broken, and the bone which grew out at the injured spot had formed a second tail with terminal fin.
59 In the memoir on the Anatomy and Physiology of the Nematoids, by Dr. Charlton Bastian, which appeared in the Philosophical Transactions for 1866, we read that even these lowly organized worms have little power of repair. Speaking of the “paste eels” (Anguilulidæ), he says, “I may state as the result of many experiments with these that the power they possess of repairing injuries seems very low. I have cut off portions of the posterior extremity, and though I watched the animal for days after, could never recognize any attempt at repair.” Perhaps, however, the season may have some influence; and Dr. Williams’s denial respecting the Naïs may be thus explained. [What is said above was written in 1868, and published in the June number of the Fortnightly Review. In the August of that year the question of reproduction of lost limbs was treated by Prof. Rolleston in his Address to the British Medical Association, in which he showed cogent evidence for the conclusion that the reproduction of limbs only exists is animals that have feeble respiration, and consequently slow vital processes.]
60 This beautiful and transparent larva reminds one in many respects of the Pike as it poises itself in the water awaiting its prey. It is enabled to do so without the slightest exertion by the air-bladders which it possesses in the two kidney-shaped rudiments of tracheæ, and which in the gnat become developed into the respiratory apparatus. The resemblance to the air-bladder of fishes is not simply that it serves a similar purpose of sustaining the body in the water, it is in both cases a rudiment of the respiratory apparatus, which in the fish never becomes developed. Weismann calls attention to an organ in the larvæ of certain insects (the Culicidæ), which have what he calls a tracheal gill, which gill has this striking analogy with the fish-gill that it separates the air from the water, and not, as a trachea, direct from the atmosphere. See his remarkable memoir Die nachembryonale Entwickelung des Muscidens, in Siebold und Kölliker’s Zeitschrift, 1864, p. 223.
61 The Variation of Animals and Plants, 1868, II. p. 272.
62 Origin of Species, 5th ed. p. 96.
63 Mr. Darwin has himself, in the following passage, stated a somewhat similar view, and rejected it: “In one sense the conditions of life may be said not only to cause variability, but likewise to include Natural Selection, for the conditions determine whether this or that variety shall survive. But when man is the selecting agent, we clearly see that the two elements of change are distinct; the conditions cause the variability, the will of man acting either consciously or unconsciously accumulates the variations in certain directions, and this answers to the survival of the fittest under nature.” (p. 168.)
64 Even in the nerve-sheaths of some Annelids there are muscles.
65 Spencer, Principles of Biology, II. 72
66 Faivre, Variabilité de l’Espèce, p. 15.
67 These luminous organs would furnish an interesting digression if space permitted it. The student is referred to the chapter in Milne Edwards’s Leçons sur la Physiologie et l’Anatomie Comparée, 1863, VIII. 94, sq. Leydig, Histologie, 1857, p. 343. Kölliker, Microscopical Journal, 1858, VIII. 166, and Max Schultze, Archiv für mikros. Anat., 1865, p. 124. My friend Schultze was kind enough to show me some of his preparations of the organs of Lempyris splendidula, from which the drawings in his memoir were made. They reminded me of the electric organs in fishes by a certain faint analogy, the trachea in the one holding the position of nerves in the other. I may remark, in passing, that it is not every phosphorescent animal that has distinct luminous organs. There is a lizard (Pterodactylus Gecko) which occasionally becomes luminous. “A singular circumstance occurred to the colonial surgeon, who related it to me. He was lying awake in bed when a lizard fell from the ceiling upon the top of his mosquito-curtain; at the moment of touching it the lizard became brilliantly luminous, illuminating the objects in the neighborhood, much to the astonishment of the doctor.” Collingwood, Rambles of a Naturalist, 1868, p. 169.
68 Max Schultze, Zur Kenntniss der electrischen Organe der Fische, 1858–9.
69 Leydig, Histologie, 1857, p. 45.
70 Owen, Anatomy of The Vertebrates, 1866, I. 358.
71 Davy, Researches, Physiological and Anatomical, 139, I. 33.
72 “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous successive slight modifications, my theory would absolutely break down.”—Darwin, Origin of Species, 5th ed. p. 227. In several passages insistence is made on this. “Natura non facit saltum” may be perfectly true; but without impugning the Law of Continuity we may urge that the Law of Discontinuity is equally true. The one is an abstract ideal conception; the other is a concrete ideal conception. According to the one, every change from rest to motion, or from one state to another, must pass through infinites; according to the other every change is abrupt. In my First Series, Vol. I. p. 327, I have shown how, on mechanical principles, every change in an organism must be abrupt. A glance at the metamorphoses of the embryo, or the stages of insect-development, will show very sudden and abrupt changes. Let me also cite Mr. Darwin against himself: “When we remember such cases as the formation of the more complex galls, and certain monstrosities, which cannot be accounted for by reversion, cohesion, etc., and sudden, strongly marked deviations of structure, such as the appearance of a moss-rose on a common rose, we must admit that the organization of the individual is capable through its own laws of growth, under certain conditions, of undergoing great modifications, independent of the gradual accumulation of slight inherited modifications.”—Origin, p. 151. See also note to § 130, further on, p. 142.
73 On the Nutrition of Monads, see the remarkable memoir by Cienkowski, in the Archiv fĂĽr mikros. Anatomie, I. 221, sq.
74 Paget, Lectures on Surgical Pathology, edited by Turner, 1865, p. 19.
75 It has recently been shown that certain Crustacea vary not only from species to species, but from genus to genus, when living in water of different degrees of saltness. By continued dilution of the salt water an Artemia was developed into another species, and this again into a Branchipus—a genus of large dimensions, with an extra abdominal segment, and a different tail; a genus, moreover, which is propagated sexually, whereas the Artemia is parthenogenetic, as a rule. See Nature, 1876, June 8, p. 133.
The exceeding importance of this fact is, that it proves specific and even generic differences to originate simply through the gradual changes of the medium and the adaptation of the organism to these new conditions. It also disproves the very common notion—adopted even by Mr. Darwin himself—that “organic beings must be exposed during several generations to new conditions to cause any appreciable amount of variation.” Again, “Natural Selection, if it be a true principle, will banish the belief of any great and sudden modification of structure.”—Comp. note to § 121, p. 132.
76 Compare Leydig, Vom Bau des thierischeu Körpers, 1864, p. 27.
77 Ferdinand Cohn, Die contractile Gewebe im Pflanzenreich, 1862. By a series of numerous well-devised experiments, Cohn found that in the stamen of the centauria a tissue exists which is excitable by the same stimula as muscle is, and which reacts like muscle, describing a similar curve when excited, and, after reaching its maximum, relaxing. Like the muscle it becomes fatigued by repeated contraction, and recovers its powers by repose. Like the muscle it may be rendered tetanic. (The researches of Dr. Burdon Sanderson and Mr. Darwin have since placed beyond a doubt the Contractility and Sensibility of certain plants.)
78 Mivart, The Genesis of Species, 1871, p. 23.
79 Dohrn, Der Ursprung der Wirbelthiere und das Princip des Functionswechsels, 1875, p 74.
80 Sigmund Mayer, Die peripherische Nervenzelle und die sympathische Nervensystem, 1876.
81 On these cells see note to § 140.
82 These terms designate the surface aspect of a transverse section, of what more correctly should be called the gray columna. See Figs. 3 to 6.
83 But this only in the higher animals. In reptiles and amphibia the medulla descends into the cervical region, as far as the second and third cervical vertebræ. This should be remembered in experimenting.
84 Foster and Balfour, Elements of Embryology, Part I., 1874. Comp. Schwalbe, art. Die Retina, in the Handbuch der Augenheilkunde of Graefe and Sämisch, 1874, I. 363.
85 The development of the olfactory lobe and bulb is similar; it need not be followed here.
86 German anatomists divide this axis into trunk and crown (Hirnstamm and Hirnmantel). There is convenience in this division. If we remove all the gray matter of the cerebrum, with all the white matter radiating from it, until we again come upon gray matter—and if we then cut the cerebellum from its descending strands of white matter—we shall have removed the crown, and leave the trunk remaining. This trunk is constituted by the corpora striata, nucleus lentiformis, optic thalami, corpora quadrigemina, crura cerebri, pons, medulla oblongata, and medulla spinalis. From this trunk all the organs of the body are directly innervated (except those innervated from the sympathetic?).
87 “On s’est préoccupé du rôle spécial que pouvaient jouer les ganglions périphériques situés dans le voisinage de certaines organes; et on a prétendu que les nerfs ne jouissaient de leur propriété d’agir sur
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