_Origin and Evolution of the Sense-organs_, 286 _Origin of Species_, the, 122 Over-individuals, 247 _Ovi di mare_, 93 Ovum, the, 268, 273 〃 , discovery of the, 58 P Paleontology, 149 Palingenesis, 231 Pampas, fossil remains in the, 112 Paris, Freethought Congress at, 319 Pathology, Virchow’s reform of, 57 Paulsen, Professor, 304 Peak of Teneriffe, Haeckel climbs the, 250 Pelagic sweepings, 70 _Pemmatodiscus gastrulaceus_, the, 277 Persephone-impulse, the, in Haeckel, 256 Philosophy and observation, 156 〃 and science, 202 Philosophy, Haeckel’s work in, 305 Phylogeny, 231 Physician, Haeckel as a, 80 〃 , qualities of the, 53 Pithecanthropus, the, 167, 308 Plankton, 70 _Plankton-studies_, 290 Plant or animal, priority of the, 205 Plastidules, 284 Political views of Haeckel, 301 Polyps, 175 Popular works, why written, 262 Potsdam, Haeckel’s birthplace, 30 Private teacher, Haeckel as, 100 Profession, choice of a, 53 Professor of Zoology, Haeckel appointed, 100 Progressive evolution, 168 Promorphology, 215 Protestant religion, character of the, 134 Protists, the, 206 Protistology, 206 Puerto del Arrecise, 250 Pupa, the, 227 R Rabl, Professor, 311 Radiolaria, the, 93, 289 〃 , shells of the, 95 〃 , system of the, 140 Rapallo, Haeckel at, 313 Realism of Haeckel, 304 _Report on the Deep-sea Keratosa_, the, 290 _Report on the Radiolaria_, the, 289 _Riddle of the Universe_, the, 22, 291, 301-7 Ritter, Paul von, donation of, 255, 299 Riviera, marine study on the, 76 Rocks, formation of the, 109 Roederer, 25 Romance nations, religion of the, 134 Rome, Freethought Congress at, 316 Roux, Professor, 249 Russia, travels in, 259 S Scandinavia, visit to, 257 Schiller, 42, 43 Schleiden, M. J., 47 Schleiermacher, 27-8 Schmidt, Dr., 307 School-days at Merseburg, 34 Schopenhauer on Darwinism, 132 Schwann, Theodor, 65 Scientific method, variations of, 48 _Scilla bifolia_, search for the, 51 Scotland, visit to, 257, 259 Sea-urchin, fertilisation of the, 257 Seebeck refuses Haeckel’s resignation, 254 Semon, 43 Sergi, Professor, 319 Sethe, Anna, 81, 100 〃 , Bertha, 26 〃 , Christian, 21, 22 〃 , Christoph, 21, 25 Siphonophores, the, 246 Social Democrats, the, 285, 301 Soul, unity of the, 161 Spain, Haeckel’s visit to, 252 Specialism in science, 48 Species, early difficulties about, 37-8 〃 idea of fixity of, 105 Sponge, nature of a, 271 Sponges, Haeckel’s study of the, 270 Spontaneous generation, Haeckel’s early opposition to, 77 Spontaneous generation, possibility of, 136 Stereometric structures, 215 Stephen, Sir Leslie, 307 Stettin, Congress of, 145 Stocks, animal, 211 Strauss, 83 Struggle for life, the, 119 _Studies of the Gastræa-theory_, the, 277 Sumatra, Haeckel in, 260 Superstition in Italy, 315 _System of the Medusæ_, the, 287