Fish hearts have two chambers, frogs three, birds and mammals four — yet the place birds really pull ahead is breathing: air sacs keep the flow moving one way through the lungs, pushing efficiency to roughly 95% against about 80% in mammals. Across the same three systems — heart, respiration and forelimb — the four groups diverge so far that each keeps a heartbeat of its own. In the forelimb the same set of bones reappears as a fin, a wing and an arm: homologous structures, not merely analogous ones.
Fish hearts have two chambers, frogs three, birds and mammals four — yet the place birds really pull ahead is breathing: air sacs keep the flow moving one way through the lungs, pushing efficiency to roughly 95% against about 80% in mammals. Across the same three systems — heart, respiration and forelimb — the four groups diverge so far that each keeps a heartbeat of its own. In the forelimb the same set of bones reappears as a fin, a wing and an arm: homologous structures, not merely analogous ones.
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