Europe, after causing a devastating epidemic at the end of the fifteenth century, established a kind of modus vivendi with its hosts, and has remained as a permanent scourge in Europe.  Other microbes, like those of cholera and plague, emigrate from the lands where they are endemic, like a horde of Tartars, and after slaying all who are susceptible disappear from inanition.  The draining of the fens has driven the anopheles mosquito from England, and our countrymen no longer suffer from ‘ague.’  Cleanlier habits are banishing the louse and its accompaniment typhus fever.

Fertility and care for offspring seem as a rule to vary inversely.  The latter is the path of biological progress, and is characteristic of all viviparous animals.  That any degree of parental attention is incompatible with the immense fecundity of the lower organisms needs no demonstration.  Such fertility is not necessary to keep up the numbers of the higher species, which find abundant food in the swarming progeny of the lower types, and are not themselves exposed to wholesale slaughter.  Speaking of fishes, Sutherland says: 

Of species that exhibit no sort of parental care, the average of forty-nine gives 1,040,000 eggs to a female each year; while among those which make nests or any apology for nests the number is only about 10,000.  Among those which have any protective tricks, such as carrying the eggs in pouches or attached to the body, or in the mouth, the average number is under 1000; while among those whose care takes the form of uterine or quasi-uterine gestation which brings the young into the world alive, an average of 56 eggs is quite sufficient.

Man is no exception to these laws.  His evolution has been steadily in the direction of diminishing fertility and increasing parental care.  This does not necessarily imply that the modern European loves his children better than the savage loves his.  It is grim necessity, not want of affection, which determines the treatment of children by their parents over a great part of the world, and through the greater part of human history.  The homeless hunters, who represent the lowest stage of savagery, are now almost extinct.  In these tribes the woman has to follow the man carrying her baby.  Under such conditions the chances of rearing a large family are small indeed.  Very different is the life of the grassland nomads, who roam over the Arabian plateau and the steppes of Central Asia.  These tribes, who really live as the parasites of their flocks and herds, depending on them entirely for subsistence, often multiply rapidly.  Their typical unit is the great patriarchal family, in which the sheikh may have scores of children by different mothers.  These children soon begin to earn their keep, and are taken care of.  If, however, the patriarch so chooses, Hagar with her child is cast adrift, to find her way back to her own people, if she can.  The grasslands are usually almost as full as they can