138 THE EVOLUTION OF SEX. 



^ 2. Facts Involved i)i Sexual Reproduction. — It is necessary, 

 at the outset, to be quite clear as to the concurrence of several 

 distinct facts in any ordinary case of sexual reproduction among 

 many-celled organisms, (i.) There is, first of all, the fact that 

 special reproductive cells are present in more or less marked 

 contrast to the ordinary cells making up the body. To this 

 antithesis we have already given due prominence. (2.) Then 

 there is the further fact, that these special 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 recognise that these dimorphic sex-cells are mutually 

 dependent, — that if the egg-cell is to develop into an organism, 

 it must first be fertilised by a male element. On the facts of 

 fertilisation, therefore, as observed in plants and animals, atten- 

 tion must now be concentrated. 



§ 3. Fertilisation in Plants. — " The Newly Discovered 

 Secret of Nature in the Structure and Fertilisation 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 fertilisation,- — for that honour 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 recognised in the markings 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 

 impossible 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 difl"erent times, nature appears to have 

 designed that no flower shall be fertilised by its own pollen." 

 A few years later (1799), Andrew Knight maintained that no 

 hermaphrodite flower fertilises itself for a per[)etuity of gene- 

 rations. 



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

 afresh by Darwin, who, in his "Fertilisation of Orchids" 

 (1862), and "Effects of Cross- and Self-Fertilisation" (1876), 

 has not only shown, with great wealth of illustration, the mani- 



