No. 509] THE CATEGORIES OF VARIATION 261 



splitting up of the progeny in various ratios, but that the 

 second and subsequent generations bred true to type, 

 thus presenting a condition just the reverse of Mendelian 

 inheritance. For instance when the mutant rubrinervis 

 was crossed with the parent type Lamarckiana the first 

 generation of hybrids were either rubrinervis or La- 

 marckianas, the proportion varying greatly in different 

 lots. The two kinds of hybrids did not split up in the 

 second generation but bred true to type. Similar results 

 were obtained by crossing several other elementary 

 species of Lamarckiana but this kind of behavior does 

 not seem to be generally characteristic of the elementary, 

 species of other forms. 



In the crossing of elementary species there is, accord- 

 ing to De Vries, one unit character which is -not mated, 

 since 



the differentiating mark is present in one of the parents and not in 

 the other. While all other units are paired in the hybrid it is not. 

 It meets with no mate and must therefore remain unpaired. The 

 hybrid of two such elementary species is in some way incomplete and 

 unnatural. In the ordinary course of things all individuals derive 

 their qualities from both parents; for each single mark they possess 

 at least two units. Practically but not absolutely equal, these two 

 opponents always work together and give to the offspring a likeness 

 to both parents. No unpaired qualities occur in normal offspring; 

 these constitute the essential features of the hybrids of species and 

 are at the same time the cause of their wide deviations from ordinary 

 rules. 



These differences between variations were predicted 

 by De Vries on the basis of his pangen theory, and in his 

 essay on ' i Intracellular Pangenesis" published in 1889 

 he expresses the opinion that fluctuating variability which 

 rests upon numerical variation of the pangens plays but 

 a minor part in the modification of species. The "art- 

 bildende" or species forming variability, is dependent 

 upon the appearance of an entirely new kind of pangen. 

 When categories of variation are anticipated a priori on 

 the basis of a theory of the constitution of living matter 

 there is naturally produced a temptation or bias towards 



