220 REVIEWS. 



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

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

 and closely interbred, these would be almost dwarfs, their 

 average 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 increasing in the succeeding generations. If self- 

 fertilization is injurious, the ill effects would be expected to 

 be cumulative. " But," instead of this, " the difference be- 

 tween the two sets of plants in the seventh, eighth, and ninth 

 o-enerations taken too^ether is less than in the first and second 

 generations together." Upon this Mr. Darwin remarks : 

 " When, however, we remember that the self-fertilized and 

 crossed j^lants are all descended from the same mother plant, 

 that many of the crossed plants in each generation were re- 

 lated, often closely related, and that all were exposed to the 

 same conditions, which, as we shall hereafter find, is a very 

 important circumstance, it is not at all surprising that the 

 difference between them should have somewhat 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 

 between two flowers of the same plant of Ipomsea, while 

 other flov/ers were self-fertilized as before. On raising seed- 

 lings from the two lots, it was found that such crossing gave 

 no superiority ; indeed, the offspring of the self-fertilized 

 flowers appeared to be rather more vigorous than the close- 

 crossed. And other experiments led to the same conclusion, 

 namely, that there was no particular benefit from cross-fer- 

 tilization on the same plant. In the other case, the cross 

 was made not only between the flowers of distinct plants, but 

 between those from different sources, and which had pre- 

 sumably grown under somewhat different conditions. For 

 instance, several flowers of the ninth generation of crossed 

 plants of Ipomaea were crossed with pollen taken from the 



