Page:Popular Science Monthly Volume 85.djvu/137

Rh the greater number of males than of females. In certain insects it is known that females only develop from fertilized eggs, and in one of these cases, viz., Phylloxera, Morgan has discovered that this is due to the fact that all the male-producing spermatozoa degenerate and that only female-producing spermatozoa become functional. Possibly experimental alterations of the sex ratio, such as Hertwig, King and others have brought about, may be explained in a similar way. At least the chromosomal theory of sex determination is so well established in so many cases and has been found to be true in so many instances where at first it seemed impossible of application, that it ought not be abandoned until unmistakable evidence can be adduced against it. For the present at least we are justified in concluding that sex is irrevocably determined at the time of fertilization.