The differences between the two forms of reproduction being thus much less than at first sight appears, we are led to inquire for the reason why the more complex and difficult process is so universal. Sexual reproduction appears to confer two benefits on organisms--(1) “When species are rendered highly variable by changed conditions of life, the free intercrossing of the varying individuals will tend to keep each form fitted for its proper place in nature, and crossing can be effected only by sexual generation”; (2) Many experiments tend to show that free and wide intercrossing induces vigour in the offspring. Darwin concludes that the reason why the germ-cell perishes if it does not unite with another from the opposite sex is simply because it includes “too little formative matter for independent existence and development.” He was led to this conclusion by the fact that the male and female germ-cells “do not in ordinary cases differ in their power of giving character to the embryo,” and also from experiments which seemed to show that a certain number of pollen grains or of spermatozoa may be required to fertilise a single seed or ovum. “The belief that it is the function of the spermatozoa to communicate life to the ovule seems a strange one, seeing that the unimpregnated ovule is already alive, and continues for a considerable time alive.” It is very remarkable to note how largely Professor Weismann’s conclusions on this subject were anticipated by this part of Darwin’s work. _Graft hybrids._--The probability that a graft may alter the character of the stock to which it is united, so that hybrid buds might be formed by budding or grafting the tissues of distinct varieties or species, would, if it became a certainty, prove the essential identity of sexual and asexual reproduction; “for the power of combining in the offspring the characters of both parents is the most striking of all the functions of sexual generation.” _Direct action of the male element on the female._--Pollen from another species is known to affect the mother-plant in certain cases. Thus pollen from the lemon has caused stripes of lemon-peel in the fruit of the orange; the peel is, of course, formed by the mother-plant, and is quite different from the part which the male element is adapted to affect--viz. the ovule. Similar cases are known among animals, as in the celebrated example of Lord Morton’s mare. _Development._--The changes by which the embryo reaches maturity differ immensely, even within the limits of the same compact group. Forms which closely resemble each other in the mature state, and are intimately related to each other, such as the various species of lobster and crayfish, etc., pass through a totally different developmental history. Hence we are led to believe in the complete independence of “each structure from that which precedes and follows it in the course of development.”