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Show 462 GENERAL RESULTS. CHAP. XII. exception of the lowest organisms this is possible only by means of the sexual elements, these consisting of cells separated from the body, containing the germs of every part, and capable of being fused coinpletoly together. It has been shown in the present voluine that the offspring from the union of two distinct individuals, especially if their progenitors have been subj ected to very different conditions, have an iminense acl vantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring fron1 one of the same parents. Anc.l this fact is amply sufficient to account for the development of tho sexual elements, that is, for the genesis of the two sexes. It is a different question why the two soxos are sometiines combined in the same individual and are sometimes s parated. As with many of the lowest plants and animals tho conjugation of two individuals which are either quite similar or in some degree different, is a common phenomenon, it seems probable, as remarked in the last chapter, that the sexes were primordially separate. The individual which receives the contents . of the other, may be called the female; and the other, which is often sinaller and n1ore locomotive, may be called the male; though these sexual naines ought hardly to be appli d as long as the whole contents of the two forms are blen<led into one. The object gained by the two s xes becoming united in the same hermaphrodite form pro ba bl y is to allow of occasional or frequ nt s lf-f rtilisation, so as to ensure the propagation of the species, more especially in the case of organisms affix d for life to the same spot. There docs not seein to be any gr at difficulty in understanding how an organism, formed by the conjugation of two individua~s whi ·h r presented tho two 0IIAP. XII. GENERAL RESULTS. 463 incipient se~es, might have given rise by budding first t o a .monmcious and then to an henna phrodi te form . and I~ the cas? of animals even without budding t~ an. hermaphrodite form, for the bilateral structur " of animals porhap · d' t ,_ h • <- .s ~n 1ca es tuat t ey were aboriginally fonned by the fuslon of two individuals. It is a more difficult problmn why soine plants and appar~nt~y all the h~gher animals, after becoming h rma~ Jhrodites, .have s1nce had their sexes re-separat d. ! his separation has been attributed by some naturalIsts to the advantages which follow from a division of physiological labour. The principle is intelligibl w~en the sa~e organ has to perform at the same tin1e diverse functions; but it is not obvious why the male and female glands when placed in different parts of the same compound or simple individual, should not perfo1~m . thei~· fu.n~tions equally well as when placed in two distinct Individuals. In so1no instances the sexes may have been re-separated for the sake of preventing too frequent self-fertilisation; but this explanation does not see1n probable, as tho same end might have b~en gained by other and simpler 1noans, for instance dichogamy. It may be that tho production of the male and fe1nale reproductive clements and th maturation of the ovules was too great a strain and expenditure of vital force for a sino-le individual to . b Withstand, if endowed with a highly complex organi-sation; and that at the same tin1e there was no need for all the individuals to proc~uce young, and consequently that no injury, on the contrary, good resulted from half of them, or the males, failing to produce offspring. There is another subject on which some light is thrown by the facts given in this volume, namely, hybridisation. It is notorious that when distinct |