Hydra, 50, 51, 52, 53, 68, 69;
  comparative study of, 204, 205, 206; 254;
  cells of, 255; 256, 257, 258, 261, 262, 263, 265, 266.

Hydrogen, 25, 27.

Hyracotherium, 96.

Ichthyornis, 99.

Ichthyosaurus, 94.

Indians, American, pictography of, 223, 224;
  of Brazil, 227;
  life of, 272.

Individual development, a resume of history of species, 63.

Inertia, 155.

Infant, human, activities of, 216.

Ingestive structures, 17.

Inheritance, 110, 131;
  biological laws of, 142;
  paternal and maternal basis of, 144; 145;
  Mendelian phenomena of, 146;
  Galton’s Law of, 147;
  laws of, in mental phenomena, 203;
  strength of, in mental traits, 232;
  physical, provides mechanism of intellect, 233.

Insects, butterflies, beetles, bees, grasshoppers, spiders, scorpions, 49;
  66;
  eggs of common house-fly, 67; 82;
  nervous mechanism of, 205;
  communities of, 207, 258-260, 267;
  nervous system of, 256, 257.

Instinct, determined by heredity, 206;
  of higher animals, 208;
  differs from intelligence in degree, 210.

Intelligence, 203;
  in mental life of communal insects, 207.

Invertebrates, lower animals devoid of backbone, 47;
  structural plan, 48;
  branches of, 49;
  groups, two layer animals, 50;
  hydra, sea-anemones, soft-polyps, 50;
  more complicated, 68;
  palaeontological materials, 82;
  evolution of lowest members, 92.

Jaguar, 101.

Jastrow, 294.

Java, 173.

Jellyfish, 81.

Jordan, David Starr, 123.

Kangaroo, 42.

Keane, 185.

Lamarck, 115, 133, 135.

Lampreys, 156.

Language, most important single possession of mankind, 218.

Laplace, 29.

Larvae, of lobster, 66;
  of insects, 67;
  of ground wasp, 207;
  of caterpillar, 259;
  of wasps, 260.

Lavoisier, 29.

Law of Recapitulation, 66;
  stated by Von Baer and Haeckel, 71.

Lemurs, 158, 160, 161, 195.

Life, what is it? 27.

Limestone, 89, 90.

Links, 99.

Linnaeus, 79, 158, 183.

Lions, 101;
  environment of, 112.

Lizard, nearest form to remote ancestor, 45.

Lobsters, 66;
  larvae of, 66.

Lyell, 80, 107, 135, 136.

MacDougal, 148.

Madagascar, 161.

Mallock, 295.

Malthus, 136.

Mammalia,
  lower orders of, 42;
  their own mode of growing up, 64;
  embryos of, 64; 97;
  members of class differ, 157, 158; 200;
  order of mentality, 203.

Mammals, 40, 43, 157;
  embryo of, 171.

Mammoth, 97.

Marmosets, 161.

Marquesas, 103.