THE 



AMERICAN NATURALIST 



HYBEIDISM AND THE BATE OF EVOLUTION 

 IN ANGIOSPERMS 



Professor EDWARD C. JEFFREY 

 Harvard University 



In responding to an invitation to contribute to the 

 morning program of the American Society of Naturalists, 

 it has seemed to me that a statement emphasizing some 

 of the morphological features of the greatest of all bio- 

 logical problems, the modus operandi of the process of 

 evolution, would be of interest to my fellow biologists. 

 The most distinguished as well as the most profound in- 

 vestigator, which our science has yet produced, Charles 

 Darwin, has unequivocally expressed the opinion in the 

 " Origin of Species," that morphology is the soul of nat- 

 ural history. As I am addressing a body of men who call 

 themselves naturalists, my theme will, I hope, not appear 

 unimportant. 



The rate of evolution has not been the same at all 

 periods of our earth's history. There is an agreement 

 among those whose knowledge of the vegetable population 

 of earlier eras makes their opinion worthy of serious re- 

 gard, that the plant kingdom in former times was in a 

 much less rapid condition of evolutionary change than in 

 the present age. Within the limits necessarily assigned 

 to me it is impossible to state all the probable causes of 

 this notable acceleration in the rate of change in plants. 

 I shall touch only upon two aspects of this problem and 

 of these I shall be able to develop but one. 



Extremely important factors in the evolution of plants 

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