OTHER THEORIES OF SPECIES-FORMING. 3 J 7 



"An astonishing result of the hybridisation between the black 

 walnut, Juglans nigra, used as the pistillate parent, and the Cali- 

 fornia walnut, /. californica, staminate parent, are walnut trees 

 which grow with such an amazing vigour and rapidity that they 

 increase in size at least twice as fast as the combined growth of 

 both parents. Many tremendous growers are got in the first gen- 

 eration, but in the second there are included some of the most 

 rapidly growing trees, perhaps, in the world. This hybrid has 

 clean-cut, glossy bright-green leaves from two to three feet long 

 with a sweet odour like that of apples, but it produces few nuts. 

 Curiously enough the result of hybridisation by using the pollen 

 of nigra on pistils of californica produces in abundance large nuts 

 of a quality superior to that possessed by either parent. 



"The famous Shasta daisy is the result of a multiple crossing 

 between an American and a European species of field daisy and 

 then between these hybrids and a Japanese form. The fragrant 

 calla, known as 'Fragrance/ is descended from a single individual 

 found by Burbank while critically examining a block of Little Gem 

 calla seedlings. He was surprised in this examination by a fra- 

 grance resembing that of violets or water-lilies ; as he had long 

 been seeking a fragrant calla, the individual giving this perfume was 

 carefully hunted out. No farther selecting was done; this plant 

 was the single ancestor of the fragrant new race. 



"And so one might go on for pages, but with slight variations in 

 detail all these pages would tell only the same story : the stimulating 

 or inducing of variability by environmental influences and by hybrid- 

 isations; the search after, and keen recognition of, promising spe- 

 cial variations ; the selection of the plants showing these variations ; 

 rearing new generations from them, repeated selection, and new 

 hybridisations to eliminate this characteristic or introduce that, 

 and on until a desirable combination is found. Then the careful 

 fixing of this form or type by repeated selection through several 

 generations. 



"But an end must be made of this. Let us, in a paragraph, simply 

 sum up the essential things in the scientific aspects of Burbank's 

 work. No new. revelations to science of an overturning character; 

 but the revelation of the possibilities of accomplishment, based on 

 general principles already known, by an unusual man. No new laws 

 of evolution, but new facts, new data, new canons for special cases. 

 No new principle or process to substitute for selection,* but a new 

 proof of the possibilities of the effectiveness of the old principle. 

 No new categories of variations, but an illuminating demonstration 

 of the possibilities of stimulating variability and of the reality of 

 this general variability as the fundamental transforming factor. 



