Darwin's _Origin of Species_ threw a new light upon instinct by showing that natural selection can operate on the subtlest modifications. It can discriminate shades of hardiness to climate, shades of intellectual acuteness, or shades of courage. It can intensify qualities which appear only in adults past bearing or in individuals congenitally incapable of propagation. Human selection, though a blunt tool in comparison with natural selection, can originate a bold and hardy race of dogs, or showy double flowers incapable of producing seed. In the second case fertile single flowers continue the race, as in the garden Stock. Darwin pointed out that the barren double flowers of the Stock answer to the workers of social bees and ants, the fertile single flowers to the functional males and females. Every modification that works to the advantage or disadvantage of the race, whether we classify it as physical, intellectual, or moral, gives scope for the operation of natural selection. The comparative psychology of small invertebrates, such as insects, is impeded by our imperfect knowledge of their nervous physiology. Introspection is here impossible; experimental physiology and pathology, which have done so much for the psychology of the higher vertebrates, almost impossible; analogy is a treacherous guide where the structures involved differ conspicuously. We have little to guide us in the psychology of insects except their behaviour, and that is often capable of a variety of interpretations. The only course is to adopt Pasteur's watchword, "Travaillons!"—the difficulties will diminish with time and labour. The Food of Green Plants.