194 CHARLES DARWIN. 



keep [true] on account of pollen from other plants ? 

 Because this may be applied to show all plants do 

 receive intermixture." (Quoted in the " Life and 

 Letters.") In 18.41, Robert Brown, the distinguished 

 botanist, advised Darwin to read Sprengel's " Secret of 

 Nature Displayed" (Berlin, 1793). The result was to 

 encourage and assist Darwin in his work on fertilisa- 

 tion of flowers by insects, and to bring about the first 

 due recognition of Sprengel's merits, long after his 

 death. 



" The Effects of Gross- and Self-fertilisation in the 

 Vegetable Kingdom." This work has a very direct 

 bearing on that last mentioned. Darwin speaks in 

 the Autobiography " of having come [in 1839] to the 

 conclusion in my speculations on the origin of species, 

 that crossing played an important part in keeping 

 specific forms constant." Later on he came to see 

 that the advantage of crossing is more direct, and 

 results from the greater vigour of the offspring over 

 those of self- fertilised plants. The object of this 

 work, published in 1876, was to prove this point by 

 experimental evidence of sufficient amount, and to 

 show in numerous cases, by measurements of height 

 or weight, or by counting the number of seeds pro- 

 duced, that cross-fe.rtilisation invariably tends towards 

 the greater vigour of offspring. 



Hence the motive cause for the marvellous 

 adaptations by which cross-fertilisation is ensured 

 was supplied. 



" Different Forms of Flowers on Plants of the Same 



