APPENDIX C 257
mythology occurred when Jupiter turned himself into a bull. A construction of this nature is not scientific, and no explanation of this kind, given of a developmental process—embryology teems with such accounts of phenomena—will bear the analysis of the microtome and microscope. A juggler may succeed in every attempt to deceive his audience into the sure belief that he has converted an old hat into a live rabbit, but the method and details of the “metamorphosis” will not permit of a closer inspection. Surely, notwithstanding the circumstance that writers of textbooks may state the metamorphosis of the trochophore into the annelid as a proved fact, there is to-day no practical embryologist prepared to maintain it ? Neither as a whole nor as to the parts of its body, neither as an organism nor as a series of organs, does the larva, or asexual generation, become changed into the worm, or sexual generation.
In 1886 the late Professor N. Kleinenberg set up the doctrine of “ development by substitution of organs.” His conclusions were based on this very instance of the developmental facts relating to the trochophore and the annelid worm. He had found that the trochophore possessed a nervous system quite distinct from that of the future worm, not homologous with this, and such a one that the future central nervous system of the worm did not arise from it, but, on the contrary, the larval one degenerated. From these and other facts, at the close of a fine and laborious investigation, he came to the conclusion that for any given organ—for example, the nervous system—in the course of the ancestral history the original structure had become insufficient, and had been replaced by a new organ arising from a different source. The old organ appeared in the development, because the stimulus it produced was needed for the production of the replacing organ. Applying this principle generally to all the organs, he concluded, finally, that in development there was a substitution of organs corresponding to that which had occurred in the history of the race.