Interpolations into Juvenile Stages
Early studies of marine plankton by Johannes Müller and others had revealed an astonishing diversity of larval forms, exquisitely designed for the conditions of their juvenile existence. These adaptations for dispersion and protection were well known to Haeckel, himself no stranger to marine exploration (Haeckel wrote the volumes on siphonophores and radiolarians for the Challenger reports). Even Haeckel could not attribute ancestral significance to all these structures, for they were clearly the transitory adaptations of floating or swimming larvae belonging to lineages whose adults had been sessile throughout their geologic history. Thus, Haeckel gathered them into his first and largest category of cenogenesis—larval adaptations interpolated into the palingenetic record, but not altering it in any other way.
Critics of the biogenetic law often cited the frequency of such interpolations as an argument for abandoning the law as useless in practice. Giard (1905), for example, gathered under the name of poecilogonie all cases known to him in which similar adults developed from strikingly different larvae. The biogenetic law, he argued, would attribute the adult similarities to convergent evolution and view the divergent larvae as signs of distinct ancestry; but such a position would be absurd for the many cases of nearly identical adults bearing all the characteristic features of their phylum. The larval differences are adaptations: "The larvae have become divergent by adapting themselves to different environments. Heredity has maintained the similarity of adults" (p. 154). Giard argued by enumeration to ridicule the biogenetic law, and affirmed that he had adopted his position only as his counter-cases accumulated: "When, about fifteen years ago, I reported the first known cases of poecilogony, the facts appeared rare and exceptional. Since then, they have been observed very often and in almost all groups of animals" (p. 155).
The standard response of recapitulationists was generous and frustrating. They simply admitted every case and remained supremely confident that none of them could render illegible a primarily palingenetic record. F. M. Balfour, England's staunchest recapitulationist, had written: "There is, so far as I see, no possible reason why an indefinite number of organs should not be developed in larvae to protect them from their enemies and to enable them to compete with larvae of other species, and so on. The only limit to such development appears to be the shortness of larval life" (1880, 2:364).
As a stronger tactic, some opponents tried to make a quantitative assessment, arguing that cenogenesis was far more common than a