Vol. 5 No. 1 



TORREYA MRv 



January, 1905 B<)iANiCAL 



QARDCN 



DISCONTINUOUS VARIATION AND THE ORIGIN 

 OF SPECIES* 



By D. T. MacDougal 



That distinct and separate qualities expressed in recognizable 

 external characters may appear suddenly, or disappear completely, 

 in a series of generations of plants, has been a matter of common 

 observation so long that it would be difficult to hunt out and fix 

 upon the first instance of record. 



The significance of such phenomena was obviously beyond 

 the comprehension of the earlier botanists and it is evident that 

 a rational recognition of the phylogenetic value of sports and 

 anomalies necessarily awaited the development and realization of 

 the conceptions of unit-characters of the minute structures which 

 are the ultimate bearers of heredity, and of the inter-dependence 

 of the two in such manner as to constitute actual entities as 

 embodied in Darwin's pangenesis, de Vries' intra-cellular pan- 

 genesis and in Mendel's investigations upon heredity. It is 

 equally apparent that a proper interpretation of the facts in ques- 

 tion, and their distinction from the results of hybridization was 

 possible only by means of the analysis of the collated results of 

 observations upon series of securely guarded pedigree-cultures, 

 in which the derivation of all of the individuals of several succes- 

 sive generations had been noted. For it is now thoroughly 

 realized that the main questions of descent and heredity and of 

 evolution in general are essentially physiological, and as such 

 their solution is to be sought in experiences with living organ- 

 isms and not by deductions from illusory " prima facie " evidence, 

 which has been so much in vogue in evolutionary polemics, nor 



* Address delivered by invitation before the American Society of Naturalists at 



I'iiiladelphia, December 28, 1904. 



[Vol. 4, No. 12, of ToRREVA, comprising pages 177-201, was issueil December 

 30, 1904.] 



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