Footnotes for Essay IV.
[94]. Häckel, ‘Ueber die Wellenzeugung der Lebenstheilchen etc.,’ Berlin, 1876.
[95]. Darwin, ‘The Variation of Animals and Plants under Domestication,’ vol. ii. 1875, chap. xxvii. pp. 344-399.
[96]. His, ‘Unsre Körperform etc.,’ Leipzig, 1875.
[97]. Brooks, ‘The Law of Heredity,’ Baltimore, 1883.
[98]. Galton’s experiments on transfusion in Rabbits have in the mean time really proved that Darwin’s gemmules do not exist. Roth indeed states that Darwin has never maintained that his gemmules make use of the circulation as a medium, but while on the one hand it cannot be shown why they should fail to take the favourable opportunities afforded by such a medium, inasmuch as they are said to be constantly circulating through the body; so on the other hand we cannot understand how the gemmules could contrive to avoid the circulation. Darwin has acted very wisely in avoiding any explanation of the exact course in which his gemmules circulate. He offered his hypothesis as a formal and not as a real explanation.
Professor Meldola points out to me that Darwin did not admit that Galton’s experiments disproved pangenesis (‘Nature,’ April 27, 1871, p. 502), and Galton also admitted this in the next number of ‘Nature’ (May 4, 1871, p. 5).—A. W. 1889.
[99]. Weismann, ‘Ueber die Vererbung.’ Jena, 1883; translated in the present volume as the second essay ‘On Heredity.’
[100]. E. Roth, ‘Die Thatsachen der Vererbung.’ 2. Aufl., Berlin, 1885, p. 14.
[101]. Jäger, ‘Lehrbuch der allgemeinen Zoologie,’ Bd. II. Leipzig, 1878.
[102]. M. Nussbaum, ‘Die Differenzirung des Geschlechts im Thierreich,’ Arch. f. Mikrosk. Anat., Bd. XVIII. 1880.
[103]. I have since learnt that Professor Rauber of Dorpat also expressed similar views in 1880; and Professor Herdman of Liverpool informs me that Mr. Francis Galton had brought forward in 1876 a theory of heredity of which the fundamental idea in some ways approached that of the continuity of the germ-plasm (‘Journal of the Anthropological Institute,’ vol. v; London, 1876).—A. W., 1888.
[A less complete theory was brought forward by Galton at an earlier date, in 1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he proposed the idea that heredity chiefly depends upon the development of the offspring from elements directly derived from the fertilized ovum which had produced the parent. Galton speaks of the fact that ‘each individual may properly be conceived as consisting of two parts, one of which is latent and only known to us by its effects on his posterity, while the other is patent, and constitutes the person manifest to our senses. The adjacent and, in a broad sense, separate lines of growth in which the patent and latent elements are situated, diverge from a common group and converge to a common contribution, because they were both evolved out of elements contained in a structureless ovum, and they, jointly, contribute the elements which form the structureless ova of their offspring.’ The following diagram shows clearly ‘that the span of each of the links in the general chain of heredity extends from one structureless stage to another, and not from person to person:—
Structureless elements {...Adult Father... } structureless elements
in Father {...Latent in Father...} in Offspring.’
Again Galton states—‘Out of the structureless ovum the embryonic elements are taken ... and these are developed (a) into the visible adult individual; on the other hand ..., after the embryonic elements have been segregated, the large residue is developed (b) into the latent elements contained in the adult individual.’ The above quoted sentences and diagram indicate that Galton does not derive the whole of the hereditary tendencies from the latent elements, but that he believes some effect is also produced by the patent elements. When however he contrasts the relative power of these two influences, he attaches comparatively little importance to the patent elements. Thus if any character be fixed upon, Galton states that it ‘may be conceived (1) as purely personal, without the concurrence of any latent equivalents, (2) as personal but conjoined with latent equivalents, and (3) as existent wholly in a latent form.’ He argues that the hereditary power in the first case is exceedingly feeble, because ‘the effects of the use and disuse of limbs, and those of habit, are transmitted to posterity in only a very slight degree.’ He also argues that many instances of the supposed transmission of personal characters are really due to latent equivalents. ‘The personal manifestation is, on the average, though it need not be so in every case, a certain proof of the existence of latent elements.’ Having argued that the strength of the latter in heredity is further supported by the facts of reversion, Galton considers it is safe to conclude ‘that the contribution from the patent elements is very much less than from the latent ones.’ In the later development of his theory, Galton adheres to the conception of ‘gemmules’ and accepts Darwin’s views, although ‘with considerable modification.’ Together with pangenesis itself, Galton’s theory must be looked upon as preformational, and so far it is in opposition to Weismann’s theory which is epigenetic. See Appendix IV. to the next Essay (V.), pp. 316-319.—E. B. P.]
[104]. Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’ München u. Leipzig, 1884.
[105]. O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung und Theilung des thierischen Eies.’ Leipzig, 1876.
[106]. Fol, ‘Recherches sur la fécondation, etc.’ Genève, 1879.
[107]. Kölliker formerly stated, and has again repeated in his most recent publication, that the spermatozoa (‘Samenfäden’) are mere nuclei. At the same time he recognizes the existence of sperm-cells in certain species. But proofs of the former assertion ought to be much stronger in order to be sufficient to support so improbable a hypothesis as that the elements of fertilization may possess a varying morphological value. Compare Zeitschr. f. wiss. Zool., Bd. XLII.
[108]. F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 69.
[109]. Arch. f. mikr. Anat., Bd. 23. p. 182, 1884.
[110]. Born, ‘Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV.
[111]. Roux, ‘Beiträge zum Entwicklungsmechanismus des Embryo,’ 1884.
[112]. O. Hertwig, ‘Welchen Einfluss übt die Schwerkraft,’ etc. Jena, 1884.
[113]. [Our present knowledge of the development of vegetable ova (including the position of the parts of the embryo) is also in favour of the view that it is not influenced by external causes, such as gravitation and light. It takes place in a manner characteristic of the genus or species, and essentially depends on other causes which are fixed by heredity, see Heinricher ‘Beeinflusst das Licht die Organanlage am Farnembryo?’ in Mittheilungen aus dem Botanischen Institute zu Graz, II. Jena, 1888.—S. S.]
[114]. E. van Beneden, ‘Recherches sur la maturation de l’œuf,’ etc., 1883.
[115]. M. Nussbaum, ‘Ueber die Veränderung der Geschlechtsprodukte bis zur Eifurchung,’ Arch. Mikr. Anat., 1884.
[116]. Eduard Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine Theorie der Zeugung.’ Jena, 1884.
[It is now generally admitted that, in the Vascular Cryptogams, as also in Mosses and Liverworts, the bodies of the spermatozoids are formed by the nuclei of the cells from which they arise. Only the cilia which they possess, and which obviously merely serve as locomotive organs, are said to arise from the surrounding cytoplasm. It is therefore in these plants also the nucleus of the male cell which effects the fertilization of the ovum. See Göbel, ‘Outlines of Classification and Special Morphology,’ translated by H. E. F. Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and Douglas H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden,’ in Berichte d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. S.]
[117]. O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des Eies.’ Jena, 1885.
[118]. This opinion was first expressed in my lecture, ‘Ueber die Dauer des Lebens,’ Jena, 1882, translated as the first essay in the present volume.
[119]. M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur Natur- und Heilkunde.’ Dec. 15, 1884.
[120]. A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.
[121]. According to the observations of Nussbaum and van Beneden, the egg of Ascaris departs from the ordinary type, but I think that the latter observer goes too far when he concludes from the form of the nuclear spindle (of which the two halves are inclined to each other at an angle) that we have before us a process entirely different from that of ordinary nuclear division.
[122]. Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti Acad. Lyncei (3) vii. 1879, Roma.
[123]. M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und XXIII.
[124]. Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.
[125]. Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss. Zool. Bd. XLII.
[126]. ‘Compt. rend.’ Tom. LIV. p. 150.
[127]. ‘Entwicklung der Dipteren.’ Leipzig, 1864.
[128]. ‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).
[129]. ‘Compt. rend.’ Nov. 13, 1882.
[130]. Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203.
[131]. Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.
[132]. ‘Science,’ vol. iv. No. 90, 1884.
[133]. Among unicellular organisms, encysted individuals are often called germs. They sometimes differ from the adult organism in their smaller size and simpler structure (Gregarinidae), but they represent the same morphological stage of individuality.
[134]. Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Bd. I. p. 777.
[135]. Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II. Abtheilung. Probably on account of the extravagant and superficial speculations of the author, the valuable ideas contained in his book have been generally overlooked. It is only lately that I have become aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to have also arrived at the same conclusion quite independently of Jäger. The latter has not attempted to work out his hypothesis with any degree of completeness. The above-mentioned observations are followed immediately by quite valueless considerations, as, for instance, that the ontogenetic and phyletic groups are in concentric ratio! The author might as well speak of a quadrangular or triangular ratio!
[136]. [Facts of the same kind are also known in the Vascular Cryptogams, Muscineae, Characeae, Florideae, etc.—S. S.]
[137]. Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’ Jena, 1883.
[138]. [I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the name of the supporting lamina of Coelenterata. See ‘Quart. Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.]
[139]. Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für wissenschaftliche Zoologie.’ Bd. XLI. 1884.
[140]. English translation, by H. Marshall Ward. Oxford, 1887, Clarendon Press.
[141]. [Such gland-cells are known in both animals and plants. See W. Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting cells of Blechnum occidentale L., and Osmunda regalis L., ‘Annals of Botany,’ vol. i. p. 49.—S. S.]
[142]. Thus in 1877 Bütschli thought that ‘the chief significance of the formation of polar bodies lies in the removal of part of the nucleus of the egg, whether this removal is effected by simple expulsion or by the budding of the egg-cell.’ ‘Entwicklungsgeschichtliche Beiträge;’ Zeitschrift für wissenschaftliche Zoologie, Bd. XXIX. p. 237, footnote.
[143]. C. S. Minot, ‘Account, etc.;’ Proc. Boston Soc. Nat. Hist. vol. xix. p. 165, 1877.
[144]. E. van Beneden and Boveri have recently, quite independently of each other, made a more exact study of these ‘Polkörperchen’ (‘Centrosoma,’ Boveri). They show that nuclear division starts from these bodies, although the mode of origin of the latter is not yet quite clear.—A. W., 1888.
[145]. The existence of polar bodies in sponges has been recently proved by Fiedler: Zool., Anzeiger., Nov. 28, 1887.—A. W., 1888.
[146]. They have now been observed in many species, so that their general occurrence in insects is tolerably certain. Compare bibliography given in Weismann and Ischikawa, ‘Weitere Untersuchungen zum Zahlengesetz der Richtungskörper,’ ‘Zoolog. Jahrbücher,’ vol. iii. 1888, p. 593.—A. W., 1888.
[147]. Van Beneden, even in his last work, considers these bodies to have only the value of nuclei; l. c., p. 394.
[148]. I purposely abstain from using a more precise term, for the complicated terminology employed in spermatogenesis hardly contributes anything to the elucidation of the phenomena themselves. Why do we not simply speak of sperm-cells and spermatoblasts, and distinguish the latter by numbers when they occur in successive generations of different form? Moreover, all the names which have been suggested for successive stages of development, can only be applied to the special group of animals upon which the observations have been made. Hence great confusion results from the use of such terms as spermatoblasts, spermatogonia, spermatomeres, spermatocysts, spermatocytes, spermatogemmae, etc.
[149]. Fol, ‘Sur l’origine des cellules du follicule et de l’ovule chez les Ascidies.’ Compt. rend., 28 mai, 1883.
[150]. Roule, ‘La structure de l’ovaire et la formation des œufs chez les Phallusiadées.’ Ibid., 9 avril, 1883.
[151]. Balbiani, ‘Sur l’origine des cellules du follicule et du noyau vitellin de l’œuf chez les Géophiles.’ Zool. Anzeiger, 1883, Nos. 155, 156.
[152]. Will, ‘Ueber die Entstehung des Dotters und der Epithelzellen bei den Amphibien und Insecten.’ Ibid., 1884, Nos. 167, 168.
[153]. [It is almost certain that this vesicle is not derived from the nucleus, but from the cytoplasm of the sperm-mother-cell. See Douglas H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden’ in Berichte der deutschen botanischen Gesellschaft, vol. v, 1887, p. 122.—S. S.]
[154]. Bütschli, ‘Gedanken über die morphologische Bedeutung der sogenannten Richtungskörperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884.
[155]. F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63.
[156]. The formation of a polar body in parthenogenetic eggs has now been proved: see [note] at the end of this Essay; see also [Essay VI].—A. W., 1888.
[157]. R. Leuckart,—article ‘Zeugung,’ in R. Wagner’s ‘Handwörterbuch der Physiologie,’ 1853, Bd. IV. p. 958. Similar observations were made by Max Schultze. These observations appear however to be erroneous, for Pflüger has since shown that the eggs of frogs never develope if the necessary precautions are taken to prevent the access of any spermatozoa to the water.—A. W., 1888.
[158]. Oellacher, ‘Die Veränderungen des unbefruchteten Keims des Hühncheneies. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXII. p. 181. 1872.
[159]. Hensen, ‘Centralblatt,’ 1869, No. 26.
[160]. Weismann, ‘Beiträge zur Naturgeschichte der Daphnoiden,’ Leipzig, 1876-79, Abhandlung VII, and ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII.
[161]. Weismann, ‘Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im Insectenei,’ Bonn, 1882, p. 106.
[162]. W. Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig, 1883.
[163]. We now know that the number of loops varies considerably in different species, even when they belong to the same group of animals (e.g. Nematodes).—A.W., 1888.
[164]. This expression is used by bee-keepers, for instance by the well-known Baron Berlepsch. Of course, it would be more accurate to say that the queen, seeing the cell of a drone, is stimulated to lay an unfertilized egg, and that, on the other hand, she is stimulated to lay a fertilized egg when she sees the cell of a worker, or that of a queen.
[165]. E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das Experiment.’ Zeitschrift für wissenschaftliche Zoologie, Bd. XVIII. p. 124. 1868.
[166]. ‘Daphniden,’ Abhandlung, vi. p. 324.
[167]. l. c., p. 150.
[168]. Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena. 1884.
[169]. I intend to publish these experiments elsewhere in connexion with other observations.
[170]. Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The Principles of Biology,’ 1864, vol. i. pp. 229, 230.
[171]. The same fact has since been ascertained in species belonging to several groups of animal.
[172]. Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73.
[173]. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107. 1873.
[174]. Valaoritis, l. c., p. 6.
[175]. I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. II. p. 219. He further states that these are the only instances hitherto known in which undoubted male cells have proved to be capable of further development when they have been unable to exercise their powers of fertilization. It must be added that the two kinds of germ-cells do not differ in appearance, but only in behaviour; the female germ-cells becoming fixed, and withdrawing one of their two flagella, while the male cells continue to swarm. But even this slight degree of differentiation requires the supposition of internal molecular differentiation.
THE SIGNIFICANCE OF SEXUAL REPRODUCTION
IN THE THEORY OF NATURAL SELECTION.
1886.