whatA meadow of embryos, each regrowing from a single stroke to an adult form, and the tree of species they belong to. Click two of them: their developments replay side by side, and a cross marks the stage where their forms first part company. The deeper their common ancestor, the earlier the cross. Lift selection and that law dissolves.

whyOntogeny recapitulates phylogeny is the most famous wrong idea in biology, and it was groping at something true: early development is conserved because later growth depends on the scaffold earlier growth built. Nothing in this piece protects early stages. The conservation emerges, and you can switch off the only thing that causes it.

howEach genome is a list of growth rules indexed by developmental stage, and a rule can only elaborate what earlier stages grew. Development has physics: a tip that grows into occupied body space fails to form. Mutations land uniformly across stages, and one dumb filter decides survival, a grown form must pass three checkpoints. An early mutation reshuffles every later collision and faces every gate; a late one faces one. The hourglass at the foot shows how far apart ancient lineages sit at each stage, beside a shadow twin evolved with the same dice and no filter. The print is a Haeckel plate, grown, not forged.