But the large fact is that all the Metazoa and the great majority of the Metaphyta, exhibit sexual reproduction, though the asex ual mode may be retained. The chief mode of multiplication in Hydra is by separating off asexually produced buds, but there is also sexual reproduction by ova and spermatozoa. But in this case the relative unimportance of the sexual method, as far as multiplication of these polyps is concerned, is emphasized by the fact that the ovary contains only one mature egg, and that there is usually only one ovary. The highest animals to show asexual multiplication regularly and in the adult stage are the Tunicata. Thus in the salps, a solitary "nurse" buds off a chain of sexual indi viduals, which eventually separate. This case is interesting since the tunicates have reached a high degree of structural complexity. Twinning by division of a single egg or embryo is a form of asexual reproduction confined to the developmental period.
(B) The term "sexual reproduction" covers several distinct facts: (I) the formation and segregation of special reproductive cells, as contrasted with those of the body or "soma" generally ; (2) the differentiation of special reproductive cells, usually the dimorphic egg-cells and sperm-cells, the latter non-viable except in fertilizing the former ; (3) the typical production of these spe cialized reproductive cells by different (male and female) organs or individuals. But to these three statements it is necessary to add several saving-clauses. (I) In many plants, most clearly in vascular cryptogams like ferns, one phase in the life-history has to do with the production of spores. These are special reproductive cells which develop without fertilization. Thus those that fail from the sporangia on the back of a fern-frond develop into small sexual prothalli (gametophytes). The egg-cell of a prothallus fertilized by a sperm-cell (or antherozoid), develops into an ordinary fern-plant (the sporophyte) ; and the life-history thus illustrates alternation of generations. This may be defined as the alternate occurrence in one life-history of two or more different forms differently produced. Spore-cells may also occur among multicellular animals, as is seen in the life-history of the liver fluke, where two larval stages (sporocysts and rediae) multiply by spore-cells, which are hardly differentiated enough to be called parthenogenetic ova, while the adult fluke reproduces by ova and spermatozoa as usual, except that self-fertilization or autog amy occurs. (2) In some animals, such as certain rotifers, the males are unknown ; in other types, e.g., summer green-flies, they may be absent for long periods ; in other cases, such as certain gall flies, the males are unnecessary even when present. In other
words, parthenogenesis (q.v.), is common. Yet this launching of an unfertilized ovum on the voyage of development should be retained under the rubric of sexual reproduction, for although there is no fertilization, there is multiplication by means of egg cells. (3) Many common animals, such as snails, earthworms and leeches, are hermaphrodite, each individual having both ovary and testis, which often ripen at different times (protandrous and pro togynous dichogamy). In the cases mentioned there is cross fertilization in spite of the hermaphroditism ; in rare cases, such as the liver-fluke and some tapeworms, there is self-fertilization or autogamy. To be distinguished from thoroughgoing her maphroditism is the normal reversal of sex in the course of the animal's life. Thus among cymothoid and epicarid crustaceans, the females pass through a male phase, though they may not function as such. According to Cunningham and Nansen all the smaller hags (Myxine) have a testis, which is subsequently replaced by an ovary. (See SEx.) Advantages of Sexual Reproduction.—Since most multi cellular plants and animals exhibit sexual reproduction, either exclusively or along with a retained asexual multiplication, the question rises as to the advantages of the sexual process. (a) That sexual reproduction is fatal to the individual in many cases, e.g., butterflies and eels, does not contradict the general proposi tion that sexual multiplication is physiologically more economical than the asexual modes when there is a large number of progeny.
(b) Although asexual multiplication may occur in complicated animals, such as tunicates, it would be attended with obvious dif ficulties in many of the highly differentiated and integrated types, such as arthropods and vertebrates.
(c) The outstanding fact implied in having specialized repro ductive units is that these have not shared in the building up of the parental "body," but have retained an organization (or equip ment of hereditary factors) continuous in quality with that of the original fertilized egg-cell from which the parent arose. They are thus not very liable to be tainted by any of the mishaps which are likely enough to befall the "body" or "soma" which bears them. This gives them an advantage over buds or fragments, for these are liable to start with such disabilities as the parental body may have acquired. Plants, such as potatoes, that are arti ficially propagated by means of cut-off pieces, are apt to lose their good qualities in the course of a number of asexually-pro duced generations.