THEORIES OF DEVELOPMENT 



hybridization in a state of nature. He has, for 

 example, seen hybrid raspberries growing wild 

 and maintaining their own in the same neighbor- 

 hood with both of their parents. The same thing 

 occurs in the case of a species of madder that 

 grows abundantly along the roadsides near Se- 

 bastopol. Mr. Burbank has seen nuts that he be- 

 lieves to be a natural cross between the pecan and 

 hickory. In a word, he believes that hybridization 

 among wild species is an exceedingly common phe- 

 nomenon, and that this is at least one of the prom- 

 inent means of developing new species and new 

 varieties upon which natural selection may work 

 differentiation of species. 



Mr. Burbank thus supplements and extends the 

 Darwinian theory, offering what seems the best 

 explanation hitherto suggested of the "origin of 

 the fittest," about which Darwin himself and his 

 chief disciples were very much in the dark. 



It should be added that Mr. Burbank 's experi- 

 ments, while showing in numberless cases the 

 possibility of the development of new varieties 

 through cross-breeding, show also the limitations 

 that nature puts upon the method by denying fer- 

 tility to hybrids that result from the crossing of 

 parents too widely divergent. For example, he 

 made an extraordinary series of cross-pollenizing 

 experiments in which the strains of many mem- 

 bers of the rose family, including the apple, the 

 pear, the mountain ash, and the rose itself, were 

 blended with those of the blackberry. Similarly 

 he crossed the raspberry and the strawberry, 



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