Hermaphroditism must be regarded as a temporary or transitional form.[40] It is found persisting in various degrees in many species--snails, earth-worms, and leeches, for example, can act alternately as what we call male and female. Other animals are hermaphrodite in their young stages, though the sexes are separate in adult life, as, for example, tadpoles, where the bisexuality of youth sometimes linger into adult life. Cases of partial hermaphroditism are very common, while in many species which are normally unisexual, a casual or abnormal hermaphroditism occurs--this may be seen in the common frog, and is frequent among certain fishes, when sometimes the fish is male on one side and female on the other, or male anteriorly and female posteriorly.[41] There would seem to be a constant tendency to escape from these early and experimental methods of reproduction, and to secure true sexual union, with complete separation of the sexes and differences in the parents. We have noticed the many instances of tiny complemental males, in connection with hermaphrodite forms, which, as Darwin states, must have arisen from the advantage ensuring cross-fertilisation in the females who harbour them. Even among hermaphrodite slugs we find very definite evidence of the advance of love; and in certain species an elaborate process of courtship, taking the form of slow and beautiful movements, precedes the act of reproduction.[42] Some snails, again, are provided with a special organ, a slightly twisted limy dart, which is used to stimulate sexual excitement.[43] What do such marvellous manifestations, low down in the ladder of life, go to prove, if not that there must be the closest identity between the development of life and the evolution of love?