3. If at the time of rutting a single ovum is detached from the ovary, and descends slowly through the genital canal (animals which bear a single offspring), fertilization taking place at the beginning of the rutting suffices to produce a female, and at the end of the rutting to produce a male, provided that the change in the condition of the ovum takes place normally during its passage through the genital canal. Both the theory as Thury stated it, and the critical remarks that have been made upon it, have been further elucidated in many subsequent works. Here Pictet, Chavannes, C. Vogt, De Philippi, and others, have taken part. Pictet believes in the uniformity of the sexual life of vegetables and animals, so that both would be subject to identical fundamental laws. The facts which stand in certain relations with the fundamental laws are numerous, and the manner in which they tend to affect those fundamental laws occasions various combinations in the variety of phenomena. For the animal kingdom Thury adduces a number of observations as the foundation of his teaching. We shall here turn our attention to some of them. We have already pointed out that in the case of the eggs of the singing birds, which are laid by turns, the young which emerges from the last strikingly smaller egg, the so-called “nest egg,” is always a male. According to the theory of Thury, the ripeness of the ovum depends also upon the place which, in the animals, it occupies in the ovary. In consequence, according to this author, it is not improbable that we shall find an irregularity in the successive production of male and female ova. If the activity of the generative apparatus of the female should be increased by any circumstance, in the case of the animals the ripening of the ovum would be accelerated. The consequence would be an earlier detachment or emptying out of the ova from the ovary. In consequence, the generative operations in the animal are of a more complex nature than in plants, a fact which is of great significance for the determination of sex. The continuous intercourse of the male with the female increases the capacity for accelerating the ripening of the ovum. According to Burdach the mother animals who do not have frequent intercourse with the male bear more females, because their ova do not, before fertilization, attain so high a degree of ripeness as to be able to develop into males. Also, it appears from observations on animals, not to be improbable that the male chooses the time of intercourse. The determination of the chosen time depends upon many influences. The causes of choice may be sought in many factors. Some of these depend upon external influences; others have internal causes. The causes may be general or personal. They may depend upon external form, or be occasioned by other phenomena of the animal world. It is always easy for these phenomena, which in their nature are of the most different kinds, to escape observation.