146 INBREEDING AND OUTBREEDING 



growth. In fact, we have found no record of intervarietal 

 crosses where delicate or weak progeny resulted. It 

 would not be useful, however, to attempt to canvass the 

 literature for all those cases in which crossing either did 

 or did not result to the advantage of the offspring. A list 

 of the crosses would alone fill a volume. It is only neces- 

 sary to point out that the value to be derived from cross- 

 ing thus made so evident gave great impetus to the study 

 of tioral structures as adaptations for cross-pollination. 

 So zealously was this line of investigation pursued, that 

 knowledge of the methods of pollination in the angio- 

 sperms soon exceeded that of any other phase of general 

 botany. The interpretation placed upon many of these 

 floral mechanisms was fantastic, to say the least, the en- 

 thusiastic claims of the workers rivalling those of zool- 

 ogists in mimicry and protective coloration. The net re- 

 sult was simply to show how widespread were means of 

 cross-pollination. It might be said to have proved that 

 cross-fertilization is an advantage ; it did not prove it to 

 be indispensable. There were too many naturally self- 

 fertilized plants for any such conclusion. 



Of all the work on the effects of crossing in pre-Men- 

 delian times, that of Darwin is the most important. With 

 it we get a new insight into the meaning of inbreeding and 

 outbreeding. Darwin was the first to see it was not the 

 mere act of crossing which was beneficial. He satisfied 

 himself on this point by crossing different flowers on the 

 same plant and different plants of similar strains. In 

 neither case was there any positive evidence of an effect. 

 But crosses between different varieties or species of 

 plants gave unmistakable signs of invigoration. In 24 

 cases out of 37, cross-fertilization increased the height 



