The feeling which makes husband and wife true companions for better and worse can grow up only in societies where the altruistic sentiments of man are strong enough to make him recognize woman as his equal, and where she is not shut up as an exotic plant in a green-house, but is allowed to associate freely with men. In this direction European civilization has been advancing for centuries, and there can be no reason to fear that it will ever be permanently diverted from the path by which alone some of the most important of its ends can be attained. When affection came to play a more prominent part in human sexual selection, higher regard was paid to intellectual, emotional, and moral qualities, through which the feeling is chiefly provoked. Later on, we shall see how great are the consequences which spring from this fact. For the present it may be enough to say that the preference given to higher qualities by civilized men contributes much to the mental improvement of the race. Dr. Stark observes that the intemperate, profligate, and criminal classes do not commonly marry; and the like is to a large extent true of persons who are very inferior in intellect, emotions, and will.[2107] Affection depends in a very high degree upon sympathy. Though distinct aptitudes, these two classes of emotions are most intimately connected: affection is strengthened by sympathy, and sympathy is strengthened by affection. Community of interests, opinions, sentiments, culture, and mode of life, as being essential to close sympathy,[2108] is therefore favourable to warm affection. If love is excited by contrast, it is so only within certain limits. The contrast must not be so great as to exclude sympathy. Great difference of age is fatal to close sympathy. Wieland noted that most people who fall in love do so with persons of about their own age;[2109] and statistics prove the observation to be correct. Men who marry comparatively late in life usually avoid too great difference in age.[2110] The foundation of this admiration and preference, modified by age, says Mr. Walker, “appears to be the similarity of objects and interests which are inseparable from similar periods of life, the association of these with a similar intensity of sexual desire, the consequent production of similar sympathy, and the resolve that it shall be permanent.”[2111] A very important factor is similarity in the degree of cultivation. It seldom happens that a “gentleman” falls in love with a peasant-girl, or an artizan with a “lady.” This does more than almost anything else to maintain the separation of the different classes, and to preserve the existing distribution of wealth among the various groups of society.