From the almost total absence of fossil evidence relative to the origin of the phyla, it follows that any explanation of the mechanism in the creative evolution of the fundamental structural plans is heavily burdened with hypothesis. This should appear as an epigraph to every book on evolution. The lack of direct evidence leads to the formulation of pure conjecture as to the genesis of the phyla; we do not even have a basis to determine the extent to which these opinions are correct.
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