VARIATIONS IN ANIMALS AND PLANTS 487 



Mendelian law may be of universal application, it is in a few cases only 

 that its facts can be clearly seen. 



In heterogeneous hybridization or the crossing of distantly related 

 forms, all sorts of results are reached, according to the nature of the 

 forms in question. In few cases does the progeny become mature, and 

 still more rarely does it prove fertile. If the plans of structure in- 

 herited from the two parents do not coalesce with reasonable com- 

 pleteness, nothing comes of the development of the progeny. Thus the 

 strawberry may be readily crossed with the raspberry. The resultant 

 plant grows and blossoms, but the divergent lines of heredity can not 

 agree on the structure of the fruit, and the plant is wholly sterile. 



In Mr. Burbank's cross of the European walnut (Juglans regia) 

 with the California walnut (Juglans Calif ornica) , the first genera- 

 tion shows a certain blending of the traits of one species with those 

 of the other. In the next generation appears every conceivable kind 

 of variation in every feature of the plant and in every function of its 

 organs. 



Among artificial hybrids of different species a few are fertile and 

 breed true to the parent stock. Among these are the Primus berry, a 

 cross between the blackberry and raspberry, and the Logan berry, a 

 chance cross between a cultivated raspberry and a California black- 

 berry (Rubus vitifolia) growing outside a garden fence in Santa Cruz, 

 California. 



These two plants might be properly called distinct species, if we 

 were not aware of the circumstances of their actual origin. But 

 hybrids rarely form distinct species in nature. European writers have 

 defined numerous hybrids among the fresh-water Cyprinidse of the 

 continent of Europe. Nothing of the sort occurs in the same group in 

 America, and the evidence for hybridization needs at least re-examina- 

 tion. 



Among American birds we have three notable cases : In the genus 

 Helminthophaga (Helminthophila Ridgway), a group of American 

 wood warblers, three nominal species have been described, each of 

 which is usually regarded not as a true species, but as a hybrid of two 

 of the common forms. One of these, called Helminthophaga cincin- 

 natiensis Langdon, is ' obviously a hybrid ' between Helminthophaga 

 pinus and Oporornis formosa. Helminthophaga lawrencei Herrick, 

 known from a few specimens, is perhaps a hybrid of Helminthophaga 

 pinus and H. chrysoptera. Helminthophaga leucobronchialis Brewster, 

 known from some scores of examples from various localities, is prob- 

 ably also a hybrid of the same two species. " It is probable," says Mr. 

 Ridgway, " that both in the case of this form and H. lawrencei, that 

 dichromatism, as well as hybridism, enters into the question of their 

 origin. While H. pinus apparently exhibits rarely a white and gray 

 phase (instead of olive and green) and H. chrysoptera as rarely a 



