Intelligence is essentially a _generalizing_ and _abstracting_ power; hence, from its very nature, it could not be _limited to a single activity_. Bestial instincts, however, though frequently so amazingly complex and ingeniously purposive as to seem the fruit of profound meditation, are, nevertheless, confined exclusively to this or that determinate ability. They operate within narrow and preëstablished grooves, from which they never swerve to any appreciable degree, being but little modifiable or perfectible by experience. Bees always construct hexagonal cells, spiders stick to the logarithmic spiral, and beavers never attempt to put their engineering skill to new uses. Instincts have but little pliancy, their regularity and uniformity being such as to make the instinctive abilities definitely predictable in the case of any given species of animal. Now, the distinctive mark of intelligence is _versatility_, that is, aptitude for many things without determinate restriction to this or that. A man who is expert in one art may, by reason of his intelligence, be equally proficient in a dozen others. The biologist may be a competent chemist, and the astronomer an excellent physicist. Michel Angelo was a sculptor, a frescoer, a painter, an anatomist, an engineer, and an architect, while Leonardo da Vinci had even more arts to his credit. To predict before birth the precise form that a man’s ability will take is an impossibility. Certain aptitudes, such as a musical gift, are no doubt inherited, but it is an inheritance which imposes no rigid necessity upon inheriter; since he is free to neglect this native talent, and to develop others for which he has no special innate aptitude. With man, the fashion in clothing and the styles of architecture vary from day to day. The brute, however, never emerges from the rut of instinct, and each generation of a given animal species monotonously reproduces the history of the previous generation. Man, on the contrary, is capable of indefinite _progress_, as the march of human cultures and civilizations shows. Gregarious animals are restricted by their instincts to determinate types of aggregation, as we see in the case of ants and bees. Hence these insect communities are unacquainted with our sanguinary revolutions which overturn monarchies in favor of republics, or set up dictatorships in place of democracies; for, fortunately or unfortunately, as one may choose to regard it, man is not limited to one form of government rather than another.