constant average ratio between the sexes adapted to the conditions of life, he gives his reasons for believing that this ratio is not determined by any inner law, but by the influence of external conditions, which act sometimes upon the parent organism, sometimes upon the egg before fertilization, sometimes upon the developing egg or embryo, and sometimes, as in the case of many plants, upon the mature organism. He also believes that the character of the influence thus exerted by external conditions has been determined for the good of each species—by natural selection.
He treats, in the first part of his paper, of those conditions which act upon the two parents in opposite ways, and he summarizes his conclusions as follows: "Each species has acquired, through natural selection, the useful property, in virtue of which any deviation from the average ratio between the sexes is corrected by an increased number of births of the deficient sex, or a decreased number of births of the sex which is in excess."
We have space for only two of the many illustrations which he quotes to prove the existence of this law, and for further proof must refer the reader to the long tables of statistics in the original paper.
As the result of nearly a million observations of the birth of colts, he shows that, as the number of mares put to a stallion in a year is increased, there is a corresponding and regular increase in the number of male colts as compared with the female colts, and he gives the following summary:
| Number of mares to one stallion. |
Number of colts. | Number of male to each one hundred female colts. | |
| Male. | Female. | ||
| 20 to 34 | 29,023 | 29,934 | 96·94 |
| 35 to 39 | 44,911 | 46,493 | 96·60 |
| 44 to 44 | 66,573 | 69,045 | 96·42 |
| 45 to 49 | 69,774 | 72,073 | 96·81 |
| 50 to 54 | 69,972 | 71,461 | 97·92 |
| 55 to 59 | 75,493 | 74,912 | 100·77 |
| 60 or more | 71,407 | 70,569 | 101·19 |
| Total | 427,153 | 434,487 | 98·31 |
In three cases where the power of parthenogenetic reproduction has been acquired as a compensation for the absence of males, the parthenogenetic eggs give rise either universally, or in the vast majority of cases, to males.
For instance, as bees destroy the males after they have been rendered unnecessary by the fertilization of the queen, they are exposed to the danger that when males are needed none may exist, and there can be no doubt that the power of parthenogenetic reproduction has been acquired by bees as a compensating adjustment.
When the nuptial flight of the queen is delayed by accident, or by the intervention of the breeder, the effect is, of course, equivalent
