A roadside museum advertises that it has fossils of meter-long insects. Explain why the respiratory system of insects would not support a large size, and therefore the existence of such fossils is highly unlikely.
Fetal hemoglobin picks up oxygen from maternal blood. If the oxygen-binding characteristics of hemoglobin in the fetus were identical to the hemoglobin of the mother, oxygen could never be transferred at the placenta to fetal circulation. What hypothesis about the oxygen binding characteristics of fetal hemoglobin would explain how fetuses get the oxygen they need?