1. Harvey hypothesized that the blood that leaves the heart in the arteries eventually returns to it by way of the veins instead of being consumed by the organs to which it flows. From this, we could predict that there must be connections between the arteries and the veins. For lack of a microscope, Harvey was unable to test this prediction, but today, microscopes enable us to observe such connections, the capillaries.

2. An automobile exhibits sensitivity and responsiveness to stimuli from the driver. Its "receptors" include such things as the ignition switch and gas pedal, and its effectors are the engine, wheels, radio, etc. It "metabolizes" gasoline and has some homeostatic mechanisms such as temperature control and battery charge. It shares a chemical unity of composition with other automobiles. It does not, however, exhibit anything analogous to growth, differentiation, reproduction, or heredity—at least not independently of its human manufacturers. The point of this is that many of the individual criteria of life also describe some nonliving entities, and life must be defined by a combination or suite of characters rather than any one unique character.

3. Had it not been for an ancestral diet that included foliage and fruits, primates might not have evolved color vision. We might still have black-and-white vision today, like most other mammals. Had the ancestral primates not captured lizards, insects, or other small moving prey, they might not have evolved stereoscopic vision. Without the need to move efficiently in an arboreal habitat, it is doubtful whether they would have evolved a prehensile hand or such a mobile shoulder joint. Without the need to remember arboreal routes to period abundances of food, such as ripening fruits, primates might not have evolved such large brains and good memories. Therefore many adaptations seen in humans today are thought to have evolved from the arboreal habitat and dietary habits of early primates.

4. Horses lack such defining characteristics of primates as flat nails and opposable thumbs, among other less obvious things such as clavicles. Some human characteristics lacking in monkeys are bipedal locomotion, tool making, articulate speech, and large brains.

5. The loss of water through sweating stimulates the sense of thirst. Thirst motivates you to drink, which restores the body’s water balance. Thus, the sense of thirst is part of a negative feedback loop that homeostatically maintains the body’s water balance within an acceptable range of its set point.

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