APPENDIX C 261
emphatically that under the conception of an antithetic alternation of generations all the phenomena observed in the development of any higher animal, from a hydroid polype to man himself, are capable of ready and simple explanation. On this view of matters it is clear why in the Nemertean life-cycle the worm should oust the Pilidium larva, why the starfish should substitute itself for the Bipinnaria, why two distinct and separate nervous systems should appear in a certain definite order in any development, why these should differ so markedly in morphological features as do that of the trochophore and that of the annelid, or as do the transient nervous apparatus of the skate development and that of the sexual generation, the skate itself. It explains why there may be two distinct nervous systems, and not three, four, or more. The doctrine of direct development is impotent in face of these and similar facts. But possibly there is a sort of explanation given by the recapitulation theory, under which every animal recapitulates more or less completely and perfectly its ancestry in the course of its individual development. There are various difficulties in this theory—in fact, it bristles with them. Like many other current beliefs, it was originally never founded in observation at all, and it is still largely advocated in spite of all observation. Were there a basis of truth in it, it ought to be easy for any embryological upholder of the doctrine to give an outline of, say, the ancestral history of the vertebrata, or of the mammalia. For the latter class, since the days of the late Professor W. K. Parker, no one except Haeckel has ever attempted to explain the facts in the light of this theory while for the former, the name of the theories supposed to be founded in recapitulation is legion. Moreover, each excludes the others.
But suppose recapitulation were really the true explanation of the facts of development, and not, as it actually is, the illusion of the human imagination: under it there would be happening daily in every development an alternation of generations, beside which that