Darwin on the Effects of Cross- and Self-Fertilization. 127 



when an extraneous stock was used as one parent, — to eminent 

 advantage, as will be seen. 



The difference in vigor between the cross-bred and the close- 

 bred progeny, as measured by early growth, was well marked 

 throughout " In the mean of the ten generations it was as 100 

 to 77. In the tenth generation it was 100 to 54, that is, five 

 cross-bred plants grew to the average height of 937 inches 

 while the close-bred were reaching the average of 504 inches. 

 This was a notably greater difference than in any previous 

 generation. But tins'" was probably accidental or anomalous; 

 far it was not led up to by successive steps. Indeed, the differ- 

 ence in the first generation was a trille greater than the average 

 of ail ten. beiniras 100 to 76. The second generation was as 

 100 to 79 ; the third as 100 to 68 ; the fourth as 100 to 86 ; 

 the fifth as 100 to 75 ; the sixth as 100 to 72 ; the seventh as 

 100 to 81 ; the eighth as 100 to 85 ; the ninth as 100 to 79 ; 

 the tenth, as already stated, 100 to 54. The general result is 

 made striking in the following illustration. 



" If all the men in a country were on an average six feet 

 high, and there were some families which had been long and 

 closely inter-bred, these would be almost 



height during ten generations being only four feet eight and 

 one-quarter inches." (p. 53.) 



It is remarkable that the difference between the close-bred 

 and the cross-bred individuals should have been as great as it 

 was in the first generation ; and, this being the case, it might 

 have been expected that the difference would have gone on 



ii. i i t - uld be expected to be cumulative. 



" But," instead of this, " the difference between the two sets of 

 plants in the seventh, eighth, and ninth generations taken to- 

 gether is les oad second generations together. 



Upon this Mr. Darwin remarks : " When, however, we remem- 

 ber that fch< : crossed plants are all descended 

 from the same mother plant, that many of the crossed plants 

 in each generation were related, often closely related, and that 

 all were exposed to the same conditions, which, as we shall 

 hereafter find, i> a \<-r\ important circumstance, it is not at all 

 surprising that the difference between them should have some- 

 what decreased in the later generations." (p. 56.) 



Further light was thrown upon these points by two kinds of 

 subsidiary experiments. In one case, the cross was made be- 

 tween two flowers of the same plant of Jpomaea, while otner 

 flowers were self- fertilized as before. On ra 

 the two lots, it was fouu - § ave no superiority : 



indeed, the offspring of the self-fertilized flowers appeared to be 

 rather more vigorous than the close-crossed. And other expen- 



