An Italian savant, Professor Sergi, has elaborated this hypothesis in considerable detail. He contends that "war continued for a long time is the origin of this phenomenon (relative sterility), not only in the absolute sense of the loss of men in battle, but also through a series of special conditions which arise simultaneously with an unbalancing of vital processes and which create in the latter a complex phenomenon difficult to examine in every one of its elements. "The biological disturbance does not derive solely from the destruction of young lives, the ones best adapted to fecundity, but also from the unfavorable conditions into which a nation is unexpectedly thrown; from these come disorders of a mental and sentimental nature, nervousness, anxiety, grief, and pain of all kinds, to which the serious economic conditions of war-time also contribute; all these things have a harmful effect on the general organic economy of nations."[108] From the combination of these losses on the battle-field and in the cradle arises what the biologist Doctor Saleeby terms "the menace of the dearth of youth." The European populations to-day contain an undue proportion of adults and the aged, while "the younger generation is no longer knocking at the door. We senescents may grow old in peace; but the facts bode ill for our national future."[109] Furthermore, this "dearth of youth" will not be easily repaired. The war may be over, but its aftermath is only a degree less unfavorable to human multiplication, especially of the better kinds. Bad industrial conditions and the fearfully high cost of living continue to depress the birth-rate of all save the most reckless and improvident elements, whose increase is a curse rather than a blessing. To show only one of the many causes that to-day keep down the birth-rate, take the crushing burden of taxation, which hits especially the increase of the upper classes. The London _Saturday Review_ recently explained this very clearly when it wrote: "From a man with £2,000 a year the tax-gatherer takes £600. The remaining £1,400, owing to the decreased value of money, has a purchasing power about equal to £700 a year before the war. No young man will therefore think of marrying on less than £2,000 a year. We are thinking of the young man in the upper and middle classes. The man who starts with nothing does not, as a rule, arrive at £2,000 a year until he is past the marrying age. So the continuance of the species will be carried on almost exclusively by the class of manual workers of a low average caliber of brain. The matter is very serious. Reading the letters and memoirs of a hundred years ago, one is struck by the size of the families of the aristocracy. One smiles at reading of the overflowing nurseries of Edens, and Cokes, and Fitzgeralds. Fourteen or fifteen children were not at all unusual amongst the county families."[110]