Note 4 — Chapter I · Isolation.

I may here most conveniently define the senses in which all the following terms will be used throughout the present discussion:—Species of isolation are, as above stated, homogamy and apogamy, or isolation as discriminate and indiscriminate. Forms of isolation are modes of isolation, such as the geographical, the sexual, the instinctive, or any other of the numerous means whereby isolation of either species may be secured. Cases of isolation are the instances in which any of the forms of isolation may be at work: thus, if a group of n intergenerants be segregated into five groups, a, b, c, d, e, then, before the segregation there would have been one case of isolation, but after the segregation there would be five such cases.