Essays Upon Heredity and Kindred Biological Problems

62

The views of most biologists who have worked at this subject agree in all essentials with that expressed above. Bütschli says (Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Protozoa, p. 148): ‘The process of encystment does not appear to have originally borne any direct relation to reproduction: it appears on the contrary to have taken place originally,—as it frequently does at the present day,—either for the protection of the organism against injurious external influences, such as desiccation or the fatal effects of impure water, etc.; and also to enable the organism, after taking up an unusually abundant supply of food, to assimilate it in safety.’ Balbiani (‘Journ. de Micrographie,’ Tom. V. 1881, p. 293) says in reference to the Infusoria, ‘Un petit nombre d’espèces, au lieu de se multiplier à l’état de vie active, se reproduisent dans une sorte d’état de repos, dit état d’enkystement. Ces sortes de kystes peuvent être désignés sous le nom de kystes de reproduction, par opposition avec d’autres kystes, dans lesquels les Infusoires se renferment pour se soustraire à des conditions devenues défavorables du milieu qu’ils habitent, le manque d’air, le dessèchement, etc.—ceux-ci sont des kystes de conservation....’

63

This is of importance in so far as single individuals might be thus compelled to encyst even when the existing external conditions of life do not require it. The substance which Actinosphaerium, for example, employs in the secretion of its thick siliceous cyst must have been gradually accumulated by means of a process peculiar to the species. We can scarcely be in error if we assume that the silica accumulated in the organism cannot increase to an unlimited extent without injury to the other vital processes and that the secretion of the cyst must take place as soon as the accumulation has exceeded a certain limit. Thus we can understand that encystment may occur without any external necessity. Similarly, certain Entomostraca (e. g. Moina) produce winter-eggs in a particular generation, and these are formed even when the animals are kept in a room protected from cold and desiccation.

64

Upon this point Professor Gruber intends to publish an elaborate memoir.

65

This view has not even been proved for Actinosphaerium, upon which Götte chiefly relies. The observations which we now possess merely indicate that the animal contracts to the smallest volume possible. Compare F. E. Schulze, ‘Rhizopodenstudien,’ I, Arch. f. mikr. Anat. Bd. 10, p. 328; and Karl Brandt, ‘Ueber Actinosphaerium Eichhornii,’ Inaug. Diss.; Halle, 1877.

66

The conception of Protozoa and Metazoa does not correspond exactly with that of unicellular and multicellular beings, for which Götte has proposed the names Mono- and Polyplastides.

67

Among the Rhizopoda encystment is only known in fresh-water forms, and not in a single one of the far more numerous marine forms which possess shells (see Bütschli, ‘Protozoa,’ p. 148); the marine Rhizopoda are not exposed to the effects of desiccation or frost, and thus the strongest motives for the process of encystment do not exist, at least among forms possessing a shell.

68

I trust that it will not be objected that the germ-cells cannot be immortal, because they frequently perish in large numbers, as a result of the natural death of the individual. There are certain definite conditions under which alone a germ-cell can render its potential immortality actual, and these conditions are for the most part fulfilled with difficulty (fertilization, etc.). It follows from this fact that the germ-cells must always be produced in numbers which reach some very high multiple of the necessary number of offspring, if these latter are to be ensured for the species. If in the natural death of the individual the germ-cells must also die, the natural death of the soma becomes a cause of accidental death to the germ-cells.

69

l. c., p. 78.

70

l. c., p. 47.

71

‘Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.

72

Id., p. 842.

73

‘Ursprung des Todes,’ p. 79.

74

l. c., p. 42.

75

‘Contributions à l’histoire des Mesozoaires. Recherches sur l’organisation et le développement embryonnaire des Orthonectides,’ Arch. de Biologie, vol. iii. 1882.

76

l. c., p. 37.

77

Julin does not enter into further details on this point, and it is not quite clear at what precise time the cells of the ectoderm atrophy; but this is irrelevant to the origin of death, since the granular mass surrounding the egg-cells at any rate belongs to the soma of the mother.

78

Leuckart finds such a great resemblance between the newly born young of Distoma and the Orthonectides, that he is inclined to believe that the latter are Trematodes, ‘which in spite of sexual maturity have not developed further than the embryonic condition of the Distoma’ (‘Zur Entwicklungsgeschichte des Leberegels,’ Zool. Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which resemble the Orthonectides in their manner of living and in their structure, Gegenbaur has stated his opinion that they belong to a ‘stage in the development of Platyhelminthes’ (Grundriss d. vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’ and regards them as much degenerated by parasitism; and Whitman—the latest investigator of the Dicyemids—speaks of them in a similar manner in his excellent work ‘Contributions to the Life-history and Classification of Dicyemids’ (Leipzig, 1882).

79

‘Dauer des Lebens;’ translated as the first essay in this volume.

80

See the first essay upon ‘The Duration of Life,’ p. 22 et seq.

81

‘Ursprung des Todes,’ p. 29.

82

l. c., p. 5.

83

See the preceding essay ‘On Heredity.’

84

The problem is very easily solved if we seek assistance from the principle of panmixia developed in the second essay ‘On Heredity.’ As soon as natural selection ceases to operate upon any character, structural or functional, it begins to disappear. As soon, therefore, as the immortality of somatic cells became useless they would begin to lose this attribute. The process would take place more quickly, as the histological differentiation of the somatic cells became more useful and complete, and thus became less compatible with their everlasting duration.—A. W. 1888.

85

See the preceding essay ‘On Heredity.’

86

See the first essay on ‘The Duration of Life.’

87

See the first essay on ‘The Duration of Life.’

88

These assumptions can be authenticated among the Infusoria. The encysted Colpoda cucullus, Ehrbg. divides into two, four, eight, or sixteen parts; Otostoma Carteri, into two, four, or eight; Tillina magna, Gruber, into four or five; Lagynus sp. Gruber, into two; Amphileptus meleagris, Ehrbg. into two or four. The last two species and many others frequently do not divide at all during the encysted condition. But while any further increase in the number of divisions within the cyst does not occur in free-swimming Infusoria, the interesting case of Ichthyophthirius multifiliis, Fouquet, shows that parasitic habits call forth a remarkable increase in the number of divisions. This animal divides into at least a thousand daughter individuals.

89

True development also takes place in the above-mentioned Ichthyophthirius. While in other Infusoria the products of fission exactly resemble the parent, in Ichthyophthirius they have a different form; the sucking mouth is wanting while provisional clasping cilia are at first present. In this case therefore the word germ may be rightly applied, and Ichthyophthirius affords an interesting example of the phyletic origin of germs among the lower Flagellata and Gregarines. Cf. Fouquet, ‘Arch. Zool. Expérimentale,’ Tom. V. p. 159. 1876.

90

Bütschli, long ago, doubted the application of the fundamental law of biogenesis to the Protozoa (cf. ‘Ueber die Entstehung der Schwärmsprösslings der Podophrya quadripartita,’ Jen. Zeit. f. Med. u. Naturw. Bd. X. p. 19, Note). Gruber has more recently expressed similar views, and in fact denies the presence of development in the Protozoa, and only recognizes growth (‘Dimorpha mutans, Z. f. W. Z.’ Bd. XXXVII. p. 445). This proposition must however be restricted, inasmuch as a development certainly occurs, although one which is coenogenetic and not palingenetic.

91

See the first essay on ‘The Duration of Life,’ p. 23 et seq.

92

See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46.

93

See the first essay on ‘The Duration of Life,’ p. 21.

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.