The most decisive difficulty, however, is the appearance of similar effects, brought about by different means, in widely divergent lines of evolution. Take as example the invention of sex as a mode of reproduction, both in plants and in animals; here are lines of evolution as divergent as could be, and yet the same complex “accident” occurs in both. Or take the organs of sight in two very distinct phyla—the molluscs and the vertebrates; “how could the same small variations, incalculable in number, have ever occurred in the same order on two independent lines of evolution, if they were purely accidental?”