170 THE POPULAR SCIENCE MONTHLY. 



or hermapbroditism ; 2, that bisexual reproduction was developed out 

 of non-sexual reproduction ; and, 3, that non-sexual reproduction is 

 but an unessential modification of the ordinary process of growth. 



4. Facts which furnish a Key to the Process of Deriva- 

 tion. — There are certain facts which throw light on each of these 

 steps, but, as might be expected, the light is far clearer on the higher 

 steps, because these were also the last taken. 



(a.) Facts lohich hear on the Last Step, viz., the Derivation of 

 Tlnisexuality from Bisexuality. — These facts are taken from both the 

 vegetable and the animal kingdom, but especially the former. They 

 are comprehended under the general term " cross-fertilization of hi- 

 sexuals?'' 



Plants. — It is a familiar fact that most plants are bisexual, i. e., 

 have both ovary and spermary (anther-cell), in the same individual 

 plant and in the same flower ; and that nearly all such cases are capa- 

 ble of self-fertilization. But Mr. Darwin has shown that, although 

 capable of self-fertilization, yet cross-fertilization — i. e., the fertiliza- 

 tion of the ovules of one flower, or, still better, of the flowers of one 

 plant by the pollen of another — produces more seeds, larger seeds, and 

 stronger seedlings ; in other words, produces better results. Now, it is 

 a law which necessarily results from the principle of the survival of the 

 fittest that Nature ever strives to secure better results. Therefore, 

 she immediately sets to work to contrive methods of insuring cross- 

 fertilization and preventing self-fertilization. The cross-fertilization 

 is insured — 1, by winds, aided by the lightness of the pollen ; and, 2, 

 by insects which carry the pollen from flower to flower. The beauty, 

 the fragrance, and the honey of flowers are undoubtedly intended pri- 

 marily to attract insects, and thus to insure cross-fertilization. But 

 this alone is not sufficient. It is necessary also to prevent self-fertili- 

 zation. This is done sometimes, as in orchids, by sticking together 

 the pollen in masses by means of a gummy substance, so that it can 

 not fly, and placing these masses entirely beyond the reach of the 

 stigma, and sometimes by the maturation of the ovules and of the pol- 

 len at entirely different periods. In these cases the plant is wholly 

 dependent upon insects for their fertilization, and we accordingly often 

 find the most curious and ingenious contrivances in the structure of 

 the flower to make sure that there be no failure in this respect. In 

 other cases self-fertilization is still more effectually prevented by^ 

 separation of the sexes in different flowers {Ifona'cia), or in different 

 plant individuals {Dioecia) — of course, winds and insects being still the 

 carriers between the two sexes. This separation of the sexes was un- 

 doubtedly a gradual process. In bisexual plants, habitually cross-fer- 

 tilized by winds or by insects, the one organ or the other became 

 aborted until first only rudiments remained, and finally even these are 

 lost and unisexuality is complete. These stages are sometimes detect- 

 able. 



