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.