The third stage belongs to the period when the unity of the Sphere is being destroyed by Strife. It is, therefore, the first stage in the evolution of our present world. It begins with “whole-natured forms” in which there is not as yet any distinction of sex or species.[625] They are composed of earth and water, and are produced by the upward motion of fire which is seeking to reach its like. Footnote 625: Cf. Plato, _Symp._ 189 e. In the fourth stage, the sexes and species have been separated, and new animals no longer arise from the elements, but are produced by generation. We shall see presently how Empedokles conceived this to operate. In both these processes of evolution, Empedokles was guided by the idea of the survival of the fittest. Aristotle severely criticises this. “We may suppose,” he says, “that all things have fallen out accidentally just as they would have done if they had been produced for some end. Certain things have been preserved because they had spontaneously acquired a fitting structure, while those which were not so put together have perished and are perishing, as Empedokles says of the oxen with human faces.”[626] This, according to Aristotle, leaves too much to chance. One curious instance has been preserved. Vertebration was explained by saying that an early invertebrate animal tried to turn round and broke its back in so doing. This was a favourable variation and so survived.[627] It should be noted that it clearly belongs to the period of Strife, and not, like the oxen with human heads, to that of Love. The survival of the fittest was the law of both processes of evolution. Footnote 626: Arist. _Phys._ Β, 8. 198 b 29 (R. P. 173 a). Footnote 627: Arist. _de Part. An._ Α, 1. 640 a 19. 117. The distinction of the sexes was an important result of the gradual differentiation brought about by the entrance of Strife into the world. Empedokles differed from the theory given by Parmenides in his Second Part (§ 95) in holding that the warm element preponderated in the male sex, and that males were conceived in the warmer part of the uterus (fr. 65). The fœtus was formed partly from the male and partly from the female semen (fr. 63); and it was just the fact that the substance of a new being’s body was divided between the male and the female that produced desire when the two were brought together by sight (fr. 64). A certain symmetry of the pores in the male and female semen is, of course, necessary for procreation, and from its absence Empedokles explained the sterility of mules. The children most resemble that parent who contributed most to their formation. The influence of statues and pictures was also noted, however, as modifying the appearance of the offspring. Twins and triplets were due to a superabundance and division of the semen.[628] Footnote 628: Aet. v. 10, 1; 11, 1; 12, 2; 14, 2. Cf. Fredrich, _Hippokratische Untersuchungen_, pp. 126 sqq.