120. _A_, Symmetrical, and _B_, asymmetrical curve of frequency 207 121. Life-cycle of _Coccidium lithobii_ 214 122. Conjugation of a Coccidium (_Adelea ovata_) 216 123. Conjugation of _Coccidium proprium_ 218 79 (repeated). The two maturation divisions of the 'drone eggs' 236 124. Alternation of generations in a Gall-wasp 245 125. The two kinds of galls formed by the species 246 126. Ovipositor and ovum of the two generations of the same species of Gall-wasp 247 127. Life-cycle of the Vine-pest (_Phylloxera vastatrix_) 249 128. Heterostylism 254 38 (repeated). A fragment of a Lichen 261 129. Aberration of _Arctia caja_, produced by low temperature 276 130. Skeleton of a Greenland Whale, with the contour of the body 313 131. Peridineæ: species of _Ceratium_ 325 LECTURE XX REGENERATION Budding and division--Every theory of regeneration in the meantime only provisional, a mere 'portmanteau theory'--Regeneration not a primary character--Volvox--Hydra--Vital affinities--Planarians--Heteromorphoses--Enemies of Hydroid-colonies--Regeneration in Plants--In Amphibians--In Earthworms--Different degrees of regenerative capacity according to the liability of the part to injury--Different results of longitudinal halving in Earthworms and in Planarians--Regeneration in Birds--The disappearance of the power of regeneration is very slow--Morgan's experiments on Hermit-crabs--Autotomy in Crustaceans and Insects--Regeneration of the lens in Triton. We have endeavoured to explain the handing on of the complement of heritable qualities from one generation to another as due to a continuity of the germ-plasm, and we assumed that the germ-cells never arise except from cells in the 'germ-track'; that is, from cells which are equipped, from the fertilized egg-cell onwards, with a complete sample of slumbering germ-plasm, and are thereby enabled to become germ-cells, and, subsequently, new individuals, in which the aggregate of inherited primary constituents implied in the germ-plasm can again attain to development. We have now to consider other cases of inheritance in relation to the same problem--the origin of their hereditary equipment. We know, of course, that new individuals may arise apart from germ-cells, that, in many of the lower animals and in plants, they may arise by budding and fission.