All divisions of history into periods are artificial in proportion as they are precise. In history there is, strictly speaking, no end and no beginning. Each event is the product of an infinite series of causes, the starting-point of an infinite series of effects. Language and thought, government and manners, transform themselves by imperceptible degrees; with the result that every age is an age of transition, not fully intelligible unless regarded as the child of a past and the parent of a future. Even so the species of the animal and vegetable kingdoms shade off one into another until, if we only observe the marginal cases, we are inclined to doubt whether the species is more than a figment of the mind. Yet the biologist is prepared to defend the idea of species; and in like manner the historian holds that the distinction between one phase of culture and another is real enough to justify, and, indeed, to demand, the use of distingui-shing names.