PLANT HYBRIDIZATION BEFORE MENDEL 209 



newly-described intermediate forms will certainly not convince him of 

 the correctness of the doctrine of descent. The more earnestly and care- 

 fully one proceeds in the exploration of truth, so much the more gain 

 will science and the theory of evolution derive from the investigation." 



(p. 463-) 



Focke disposes of the question whether a plant pollinated from 

 two sources could produce double-pollinated seeds, in the follow- 

 ing words : 



"By analogy with animal fertilization-phenomena, it is to be regarded 

 as unquestionable, that every single ovule can only be fertilized by a 

 single pollen tube. It is a fact that, in all experiments carried out with 

 scientific precision, no hybrid has ever been obtained, in which the 

 operation of more than one parental species was to be recognized, no 

 matter how many kinds of pollen might be placed upon the stigmas of 

 the maternal flowers." (p. 448.) 



On the basis of the available data, Focke undertook to formu- 

 late a series of statements or rules, embodying the laws of the 

 behavior of hybrids so far as the then existing information made 

 it possible to do so. This was the first direct attempt after Nageli, 

 among the hybridizers of the older school, to formulate a com- 

 plete, coherent statement of principles from the extensive body of 

 data connected with hybridization. The five laws or principles 

 which Focke laid down are as follows : 



1. "All individuals derived from the crossing of two pure species or 

 races are, when produced and grown under like conditions, as a rule 

 completely alike, or are scarcely more different from one another than 

 specimens of one and the same pure species are accustomed to he." 

 (p. 469.) 



As corollary to this statement, the following is appended: 



"Least difficult to answer is the question, concerning which it has been 

 most strenuously contended, namely, that of the greater influence of 

 the one or the other sex on the type of the progeny. The hybrids of the 

 two species or races, A and B, are like each other, indifferently whether 

 A was the male or the female parent-species in the cross. ... It is deter- 

 mined through numerous experiments rather that in the plant kingdom 

 in general, in the case of true species, the form-determining power of 

 the male and the female elements in the cross are completely like one 

 another." (pp. 469-70.) 



Focke modified this statement, however, by saying that : 



"Like all other rules of hybridization, so likewise is that of the simi- 

 larity of both products of crossing not without exceptions. It is never- 

 theless self-evident that a perchance observed dissimilarity can be re- 

 garded as conditioned by the stronger operation of the male or the fe- 

 male element only when the experiments have been instituted in like 

 manner, and when they, by several times repetition, have always led to 

 the same result." (p. 470.) 



