i 3 o THE EVOLUTION OF SEX. 



reproductive cells are dimorphic; that they, and the organisms which 

 produce them, are distinguishable as male and female. This has been 

 the main theme of the two preceding books. (3) Lastly, we have to 

 recognize that these dimorphic sex-cells are mutually dependent, — 

 that if the egg-cell is to develop into an organism, it must first be 

 fertilized by a male element. On the facts of fertilization, therefore, 

 as observed in plants and animals, attention must now be concen- 

 trated. 



III. Fertilization in Plants.— " The Newly Discovered Secret 

 of Nature in the Structure and Fertilization of Flowers," so ran the 

 title of a work published by Conrad Sprengel in 1793, embodying his 

 pioneer investigations on a now familiar field. Though not indeed 

 the first to point out the importance of insects in relation to fertiliza- 

 tion — for that honor appears to belong to Kolreuter (1761) — Sprengel 

 laid sure foundations, now somewhat hidden by the superstructure 

 which Darwin and others have built. To Sprengel' s eyes, the many 

 ways in which the nectar is protected from rain seemed full of 

 " intention." He recognized in the mafkings of the petals illumined 

 finger-posts to lead insects to the hidden hoards; and he further 

 demonstrated that in some bisexual flowers it was physically impos- 

 sible for the pollen from the stamens to pass to the tips of the carpels. 

 His general conclusion, freely stated, was, that "since a large number 

 of flowers have the sexes separate, and probably at least as many 

 hermaphrodites have the stamens and carpels ripening at different 

 times, Nature appears to have designed that no flower shall be ferti- 

 lized by its own pollen." A few years later (1799), Andrew Knight 

 maintained that no hermaphrodite flower fertilizes itself for a perpe- 

 tuity of generations. 



Sprengel's secret of Nature had, however, to be set forth afresh 

 by Darwin, who, in his "Fertilization of Orchids" (1862) and " Effects 

 of Cross- and Self- Fertilization" (1876) has not only shown, with 

 great wealth of illustration, the manifold devices for insuring that the 

 unconscious insects carry the fertilizing pollen from one flower to 

 another, but has also emphasized the beneficence of cross-fertilization 

 for the health of the species. "Nature tells us," he says, "in the 

 most emphatic manner that she abhors perpetual self-fertilization." 

 Hildebrand, Hermann Miiller, Delpino, and others, have, with con- 

 summate patience of observation, further traced out the secrets of 

 Nature in this relation; and the student may be referred to Professor 

 D'Arcy Thompson's valuable edition of Muller's "Fertilization ot 

 Flowers," Sir John Lubbock's " Flowers in Relation to Insects," and 

 the classic works of Darwin. Reference must, however, also be made 



