Darwin, and After Darwin, Volume 3 of 3 / Post-Darwinian Questions: Isolation and Physiological Selection - George John Romanes

Let A and B be two intergenerating groups in which segregate fecundity is first beginning. Of the hybrids, AB will be more fertile with A, and BA with B, than vice versa. The interbreeding of AB with A will eventually modify sexual characters of A by assimilating it to those of AB, while the interbreeding of BA with B will similarly modify sexual characters of B by assimilating it to those of BA. Consequently, A will become more and more infertile with B, while B becomes more and more infertile with A. Fewer and fewer hybrids will thus be produced till mutual sterility is complete.

To sustain this hypothesis it would be needful to prove experimentally, (1) that hybrid forms AB are more fertile with A than with B, while hybrid forms BA are more fertile with B than with A [or, it may be possible that the opposite relations would be found to obtain, viz. that AB would be more fertile with B, and BA with A]; (2) that, if so, effect of intercrossing AB with A is to make progeny more fertile with A than with B, while effect of intercrossing BA with B is to make progeny more fertile with B than with A.

Such experiments had best be tried with species where there is already known to be a difference of fertility between reciprocal crosses (e.g. Matthiola annua and M. glabra, see Origin of Species, p. 244).

INDEX

A.
Allen, Mr. J. A., on variation under nature, [34].
Amixia, [12]-[28], [110]-[115], [117]-[133].
Apogamy, [5], [6], [10], [18], [28].
B.
Belt, on physiological selection, [44].
Berg Sagnitz, Count, on self-fertilization and variability, [177].
Breeding, separate and segregate, [5].
Butterflies of polar regions and Alps, [133].
C.
Catchpool, Mr., on physiological selection, [44], [137].
Cross-infertility, [46];
and varietal divergence, [82];
and diversity of life, [169];
and stability, [170];
and specific differentiation, [170];
in domesticated cattle, [170];
testing for, [172];
Fritz Müller on, [174].
D.
Darwin, Charles, on isolation, [2], [106];
on diversity under nature, [31];
on the fertility of varieties, [50];
on the origin of cross-infertility, [51];
on distribution, [68];
on prepotency, [89];
on geographical isolation, [101], [108];
on methodical selection, [102];
on modification in large areas, [103];
on the swamping effects of intercrossing, [105];
on independent variability, [109];
on domestic animals, [110].
Delbœuf, law of independent variability, [13].
Differentiation under natural selection, [37].
Diversity of life and cross-infertility, [169].
Domesticated cattle and cross-infertility, [170], [172].
E.
Evidences of physiological selection, [62].
Evolution, monotypic and polytypic, [21], [75], [102], [107], [112], [129].
Experimental research in physiological selection, [85].
F.
Fertility of domesticated varieties, [172].
Focke, Herr, on hybridization, [175].
G.
Galton, Mr. Francis, law of regression, [39].
General conclusions, [144].
Geographical distribution and physiological selection, [65].
Giard, M., on apogamy, [14].
Grabham, Dr., on mollusca of Madeira, [135].
Gulick, Rev. J., on natural Selection as a mode of isolation, [9];
on divergence, [11];
on segregate breeding, [19];
on geographical distribution, [27];
on the prevention of intercrossing, [127];
on Mr. Wallace's criticisms, [151].
H.
Herbert, on hybridization, [173];
advance on his position, [174].
Herdman, Prof., on physiological isolation, [123].
Historical sketch of opinions on isolation, [101].
Homogamy, [5], [6];
forms of, [7], [19], [29].
Hybridization, Herbert on, [173];
in plants, [175].
Hypothesis, additional, concerning physiological selection, [178].
I.
Independent variability, [12]-[29].
Isolation, defined, [2];
forms of, [3], [6];
geographical, [3];
discriminate and indiscriminate, [5];
physiological, [9], [41], [58];
its importance, [39];
sketch of opinions on, [101];
general conclusions, [144];
Seebohm on, [173].
J.
Jordan, M., on cross sterile varieties of plants, [86];
his researches summarized, [87].
K.
Kerner, Prof. A., on prepotency, [176].
L.
Lankester, Prof. Ray, on divergent evolution, [15].
Le Conte, Prof., on fossil snails of steinheim, [95];
on isolation, [129].
Livingstone, Dr. David, Quoted, [123].
M.
Meldola, Prof., on difficulty from intercrossing, [121].
Misunderstandings of Physiological selection, [59].
Monotypic evolution, see Evolution.
Morgan, Prof. Lloyd, on sterility, [56];
on isolation, [128].
Moulton, Mr. Fletcher, an examination of Mr. Wallace's calculations on physiological selection, [157].
Müller, Fritz, on cross-infertility, [174].
N.
Nägeli, on isolation, [76];
on synoicy, [78], [82].
Natural selection, a form of discriminate isolation, [9], [10], [23];
leads to monotypic evolution, [24]-[29];
difficulties of, [41], [51].
P.
Panmixia, [12].
Physiological selection, [9], [41];
summarized, [58];
misunderstandings of, [59];
evidences of, [81]-[119];
and Weismannism, [169];
additional hypothesis, [178].
Polytypic evolution, see Evolution.
Prepotency, [89];
importance of, [176].
S.
Schmidt, Prof. Oscar, on domesticated cattle, [171].
Seebohm on isolation, [173].
Segregation, [28].
Selection, physiological, see Physiological selection.
Self-fertilization and variability, [177].
Snails of Sandwich Islands, [16], [130];
fossil of Steinheim, [95].
Specific differentiation and cross-infertility, [170].
Stability and cross-infertility, [170].
Synoicy, [78].
T.
Topographical distribution and physiological selection, [74];
of varieties, [81].
Transformation, serial and divergent, [21], [121].
V.
Variability and self-fertilization, [177].
Variation in birds, [34].
Varieties, topographical distribution of, [81].
W.
Wagner, Maritz, [3];
on geographical isolation, [76];
quoted, [103];
law of migration, [111].
Wallace, Mr. A. R., [3], [17];
quoted, [34], [47], [51], [57],[130]-[136];
criticized by Gulick, [152].
Weismann, Prof., on geographical isolation, [76], [114]-[118].
Weismannism and physiological selection, [169].

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ANESAKI, M.