The several parts which are homologous, and which, at an early embryonic period, are identical in structure, and which are necessarily exposed to similar conditions, seem eminently liable to vary in a like manner:
Homologoak eta, hasierako enbrioi-aldian, egituraz berdin-berdinak diren eta nahitaez baldintza beren pean izaten diren gorputzeko atalek antzera batean aldatzeko joera handia dutela ematen dute; hau ikusten dugu: