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.”
The functional independence of the elements or units of the body. Variability and inheritance.—Variability generally results “from changed conditions acting during successive generations.” The influence is exerted on the sexual system, and if extreme, impotence tends to be produced. Bud-variation proves that “variability is not necessarily connected with the sexual system.” The inherited effects of use and disuse of parts imply that the changes in the cells of a distant part of the body affect the reproductive cells, so that the being produced from one of these cells inherits the changes. “Nothing in the whole circuit of physiology is more wonderful.”
“Inheritance is the rule and non-inheritance the anomaly.” Inheritance follows laws, such as the tendency for a character to appear at corresponding ages in parent and offspring. Reversion “proves to us that the transmission of a character and its development ... are distinct powers.” Crossing strongly induces reversion. “Every character which occasionally reappears is present in a latent form in each generation.”
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PANGENESIS.