138 THE CONDOR Vol. XIV The geometrical ratio of reproduction of plants and animals is large enough to necessitate an increase in numbers were it not for adverse circumstances. For example: The female of each pair of quail, judging from records, lays an average of twelve to fifteen eggs. Various dangers, however, probably pre- vent the hatching of more than an average of ten young. If all of these young should survive and reproduce, at the end of the second year there would be 132 quail for every original pair. But we know that this is not the case, but that there is usually about the same number each year. This means that the death rate must equal the birth rate, and, in the case of the California valley quail, the death rate must be some five times as great as the normal minimum popu- lation. Or, in other words, the life rate, or rate of survival, must be only 2 out of every 130 quail. Taking a covey of 100 quail, probably at least 40 of that number would 56O $20 2OO 120 Fig. 58 average a brood of ten young each year. This would mean that just after the hatching season, there would be something like 500 quail where there had been 100. At the opening of the next breeding season this covey, under natural conditions, would have been reduced to the 100 again. Evidently there- fore, there would have been a mortality of about 400. There are a great many factors to account for this immense mortality, chief among them being, under natural conditions, lack of food supply and destruction by predatory mammalg and birds. If we make a hypothetical curve, the points to be brought out are made intelligible. If along the left-hand side of the graph are plotted the numbers of individuals, and along the bottom, the months of the year, the maximum and minimum number would form a curve such as is seen in A. The minimum numbers can reasonably be expected to exist just before the eggs are hatched,
