It must be said also, in the second place, that the natural prevalence of monogamy as the normal type of sexual relationship by no means excludes variations.  Indeed it assumes them.  “There is nothing precise in Nature,” according to Diderot’s saying.  The line of Nature is a curve that oscillates from side to side of the norm.  Such oscillations inevitably occur in harmony with changes in environmental conditions, and, no doubt, with peculiarities of personal disposition.  So long as no arbitrary and merely external attempt is made to force Nature, the vital order is harmoniously maintained.  Among certain species of ducks when males are in excess polyandric families are constituted, the two males attending their female partner without jealousy, but when the sexes again become equal in number the monogamic order is restored.  The natural human deviations from the monogamic order seem to be generally of this character, and largely conditioned by the social and economic environment.  The most common variation, and that which most clearly possesses a biological foundation, is the tendency to polygyny, which is found at all stages of culture, even, in an unrecognized and more or less promiscuous shape, in the highest civilization.[371] It must be remembered, however, that recognized polygyny is not the rule even where it prevails; it is merely permissive; there is never a sufficient excess of women to allow more than a few of the richer and more influential persons to have more than one wife.[372]

It has further to be borne in mind that a certain elasticity of the formal side of marriage while, on the one side, it permits variations from the general monogamic order, where such are healthful or needed to restore a balance in natural conditions, on the other hand restrains such variations in so far as they are due to the disturbing influence of artificial constraint.  Much of the polygyny, and polyandry also, which prevails among us to-day is an altogether artificial and unnatural form of polygamy.  Marriages which on a more natural basis would be dissolved cannot legally be dissolved, and consequently the parties to them, instead of changing their partners and so preserving the natural monogamic order, take on other additional partners and so introduce an unnatural polygamy.  There will always be variations from the monogamic order and civilization is certainly not hostile to sexual variation.  Whether we reckon these variations as legitimate or illegitimate, they will still take place; of that we may be certain.  The path of social wisdom seems to lie on the one hand in making the marriage relationship flexible enough to reduce to a minimum these deviations—­not because such deviations are intrinsically bad but because they ought not to be forced into existence—­and on the other hand in according to these deviations when they occur such a measure of recognition as will deprive them of injurious influence and enable justice to be done to all the parties concerned.  We too