PLANT HYBRIDIZATION BEFORE MENDEL 335 



units recur in numerous, many of them in numberless, organisms, and 

 in allied species the common part of the character is built up of the 

 same units." (ed, 1910, p. 33; ed. 1889, pp. 31-2.) 



The following statement is, perhaps, especially important as 



indicating De Vries' attitude at a period considerably antecedent 



to his experimental investigations upon unit-factors of the Men- 



delian type : 



"The hereditary factors, of which the hereditary characters are the 

 visible signs, are independent units which may have originated sepa- 

 rately as to time, and can also be lost independently of one another.'' 

 (ed. 1910, pp. 33-4; ed. 1889, pp. 31-2.) 



The fact is called attention to that, although the factors are 



independent to such a degree that each may of itself become 



weaker and even disappear completely, yet they are, as a rule, 



united into smaller or larger groups, in which they act at least 



in co-ordinate fashion, so that : 



"when external influences, such as a stimulus to gall-formation, bring 

 a definite character into dominance, the entire group to which it belongs 

 is usually set into increased activity." (ed. 1910, p. 34; ed. 1889, p. 32.) 



b. Correns, C. 



Regarding his discovery of Mendel's paper, and the inception 

 of his work with Pisum^ Professor Correns reports as follows ; 

 (letter of January 23, 1925) : 



"You ask further concerning the re-discovery of the Mendelian Laws. 

 I cannot add much to what I have contributed in the Mendel issue of 

 the 'Naturwissenschaften.' It will, in the meantime, certainly have reached 

 your hands. The operation of a principle was soon 'found in the case 

 of peas and maize. I was able accordingly soon to proceed systematically 

 in the experiments, as the two genealogies for peas in my first con- 

 tribution show. I did not come at first upon the explanation of the 

 regular relationship [Gesetzmassigkeit], but I likewise, however, did 

 not seek intensively after it. For I wished, for various reasons, to first 

 finish an extensive book upon the sexual propagation of the foliose 

 mosses, upon which I had worked for years. I then wished first to push 

 intensively the elaboration of the investigations on xenias and hybrids, 

 which had been carried on at the same time since 1894. The printing 

 of the book lasted until in August 1899; then I was able to devote 

 myself earnestly to the genetics researches. The date of the day upon 

 which, in the autumn (October) of 1899, I found the explanation, I no 

 longer know ; I do not make note of such matters. I only know that it 

 came to me at once 'like a flash,' as I lay toward morning awake in bed, 

 and let the results again run through my head. Even as little do I 

 know now the date upon which I read Mendel's memoir for the first time ; 

 it was at all events a few weeks later. I then first made ready for the 

 piress the contribution on xenias in maize. In it, it is already pointed 



