THE GEXESIS OF SEX. 175 



transitional stages still in existence, almost certain, that in other cases 

 sexuality was reached by a slower process and at a later period. It is 

 this slower jDrocess which I now wish to trace in outline : 



{a.) Fission. — In the loAvest animals the individual cells which form 

 their structure are almost wholly independent. The independent life 

 of the cell is strong, the common life of the aggregate is feeble. By 

 continued cell-multiplication, the aggregate, becoming too large to be 

 held together by the common life, divides. Thus arises the lowest 

 form of reproduction, viz., by fission. Many lower animals still prac- 

 tice this mode. 



{b.) Budding on Any Part. — In the next step excess of growth 

 occurs on any part indifferently, gives rise to a tubercle which grows 

 into a bud, assumes the structure of the parent stem, and finally sepa- 

 rates to become a new individual. This is higher than the last, be- 

 cause the original individual is not sacrificed, but only a part separated. 

 Many larval medusa3 and many polyps still practice this mode. 



(e.) Budding on a Specicd Bart. — In the last case the budding oc- 

 curs in any part. In the next step a particular part is selected, and to 

 it is assigned the function of forming buds which form new beings. 

 Many larvse of medusse belong to this category ; for they bud only on 

 the mouth-disk. This is a higher form than the last, inasmuch as the 

 assignment of a function to a particular place, or localization of a 

 function, is an invariable stej) in evolution, and always attended with 

 better results. 



(d.) Special Budding Organ, internal. — The next step was probably 

 the relegation of the function of producing buds to an internal organ, 

 as being far safer and more certain of success, which organ thus be- 

 comes by position and function strongly analogous to an ovary. This 

 is the case in larval aphids. The reproductive organ of these larvai 

 has been regarded by some as an ovary, by others as an internal bud- 

 ding organ. It is certainly not a true ovary, but rather perhaps an 

 organ uniting the yet undiffei'entiated functions of ovary and sperm- 

 ary, an organ producing cells having the properties of both germ-cells 

 and sperm-cells, and therefore capable of directly forming an embryo 

 by cell-multiplication. 



(e.) Differentiation of Sexual Elements. — The interior reproductive 

 organ last described next forms two kinds of cells which by conjuga- 

 tion produce the ovum ; the sexual elements are now differentiated, but 

 not yet the sexual organs. It is not absolutely certain that this con- 

 dition actually exists in any species now living ; but it is jwohable that 

 it does. According to Kleinenbei-g,* the reproductive organ of the 

 hydra produces both ovules and spermatozoids. In many mollusks 

 and polyps the separation of the ovary and spermary is not yet made 

 out. In some gasteropods the epithelial cells of the oviduct seem to 

 become mother-cells, in which are produced spermatozoids. The sepa- 

 » " Annals and Magazine of Natural History," vol. ii., p. 351, 1818. 



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