PLANT HYBRIDIZATION BEFORE MENDEL 333 



The fact is emphasized that, in both the plant and the animal 

 kingdoms : 



"The independent varying of single characteristics forms the rule, 

 while the combined variation of them is the exception." (ed. 1910, p. 21 ; 

 1889, p. 17.) 



De Vries then asserts, in a significant and for the time rather 



remarkable sentence, that : 



"In most cases it cannot be decided whether the germ attribute is de- 

 termined by a single hereditary character or by a small group of them." 



The section closes with a significant comment upon one phe- 

 nomenon, which, he says, 



"greatly complicates the study of hereditary characters," viz., the fact 

 "of their being commonly united in smaller or larger groups which be- 

 have like units, the single members of the groups usually appearing 

 together." 



De Vries remarks upon the fact that different authors, such as 



Darwin and Nageli, have also strongly emphasized this point, but, 



he adds, the working-out of the theory in detail is rendered diflfi- 



cult by the fact that : 



"In many cases it will obviously be extremely difficult to decide 

 whether one is dealing with a single hereditary character, or with a small 

 group of them." (ed. 1810, pp. 23-4; ed. 1889, pp. 21-2.) 



It will be especially interesting to quote rather fully from the 

 discussion on the matter of unit-characters in hybrids : 



"In summarizing briefly what has been said, we see that experiments 

 and observations on the origin and fixing of variations teach us to 

 recognize hereditary characters as units with which we can experiment. 

 They teach us further that these units are miscible in almost every pro- 

 portion, most experiments really amounting merely to a change in this 

 proportion." (ed. 1910, p. 27; ed. 1889, p. 24.) 



"The above considerations are verified in a striking manner by experi- 

 ments in hybridization and crossing. In no other connection does the 

 conception of a species as a unit made up of independent factors stand 

 forth so clearly. Everyone knows that the hereditary character of two 

 parents may be mixed in a hybrid. And the excellent experiments of 

 man3^ investigators have taught us how, in the descendants of hybrids, 

 an almost endless variation can be observed, which is essentially due to 

 a mixing of the characteristics of the parents in a most varied manner." 

 (ed. 1900, p. 27; ed. 1889, p. 25.) 



"The hybrids of the first generation have quite definite characteristics 

 for each pair of species. If one produces a hybrid of two species, which 

 previous investigators have already succeeded in crossing, he can, as a 

 rule, rely on the description given of it tallying exactly with the newly 

 produced intermediate form, if the hybrid is fertile without the help 

 of its parents, and if its progeny are grown through a few generations 





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