270 SUMMARY OF MEASUKEMENTS. CHAP. VII. 



during several generations profit only to a small extent 

 by a cross with intercrossed plants of the same stock 

 (as in the case of Dianthus), in comparison with the 

 effects of a cross by a fresh stock. Plants of the same 

 stock intercrossed during several generations (as with 

 Petunia) were inferior in a marked manner in fertility 

 to those derived from the corresponding self- fertilised 

 plants crossed by a fresh stock. Lastly, certain plants 

 which are regularly intercrossed by insects in a state 

 of nature, and which were artificially crossed in each 

 succeeding generation in the course of my experi- 

 ments, so that they can never or most rarely have 

 suffered any evil from self-fertilisation (as with Esch- 

 scholtzia and Ipomoea), nevertheless profited greatly 

 by a cross with a fresh stock. These several cases 

 taken together show us in the clearest manner that it 

 is not the mere crossing of any two individuals which 

 is beneficial to the offspring. The benefit thus derived 

 depends on the plants which are united differing in some 

 manner, and there can hardly be a doubt that it is 

 in the constitution or nature of the sexual elements. 

 Anyhow, it is certain that the differences are not of an 

 external nature, for two plants which resemble each 

 other as closely as the individuals of the same species 

 ever do, profit in the plainest manner when inter- 

 crossed, if their progenitors have been exposed during 

 several generations to different conditions. But to this 

 latter subject I shall have to recur in a future chapter. 



TABLE A. 



We will now turn to our first table, which re- 

 lates to crossed and self-fertilised plants of the same 

 stock. These consist of fifty-four species belonging to 

 thirty natural orders. The total number of crossed 

 plants of which measurements are given is 796, and 



