[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 für 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.)