PROCEEDINGS 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VII, pp. 301-333 March 19, 1906. 



THE VITAL FABRIC OF DESCENT. 



By O. F. Cook. 



introduction. 



More fundamental than any other problem relating to or- 

 ganisms is the question of the method by which their develop- 

 ment has been accomplished. Any progress in this direction 

 places us nearer to an understanding of the real nature and 

 essential conditions of organic existence. To solve the prob- 

 lem, or even to approach a solution, requires a choice to be 

 made among the infinity of biological data which science has 

 already amassed, to say nothing of the still greater multitudes 

 of unrecorded facts which encounter us on every side. With- 

 out a choice of clues or a criterion of evidence, our search is 

 unscientific, hopeless wandering, with every probability of fail- 

 ure and no prospect of success. 



The history of evolution has furnished, even in its first half- 

 century, ample evidence of the truth of this fact. The first 

 step toward a causal explanation still remains to be taken ; in- 

 deed, we have not yet decided which way to face in taking it; 

 whether to seek the causes of evolution in the environment or 

 in the organisms themselves. The present paper gives reasons 

 for believing that the chief agency of evolution is to be found 

 in the association of organisms into interbreeding species, and, 

 not in the external conditions, nor in the isolation of individual 

 variations. A species is not a mere aggregation of similar in- 



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