INTRODUCTION 1 5 



his view, all species consist of a sum of ' hereditary qualities ' ; 

 very few, or none of these, are peculiar to any one species, the 

 character of which is determined by the way in which they are 

 combined. The same quality recurs in many species, but in 

 different combinations. 'We constantly see how one and the 

 same hereditary quality, or how a definite small group of such, 

 may be combined with all kinds of other hereditary qualities ; 

 and how the different characters of individual species are due to 

 the extreme variety of these combinations.'' The different organs 

 of a species stand in the same relation to one another in this 

 respect, as do the different species themselves. They exhibit 

 the same qualities, but in different combinations. The individual 

 qualities which constitute a species • can almost all vary inde- 

 pendently of each other,' and can therefore be increased even by 

 artificial selection according to the fancy of the breeder, without 

 requiring a corresponding change in the remaining qualities of 

 the species. But the qualities too are ' miscible in almost any 

 proportion," as experiments in hybridising are intended to show : 

 ' in no other way can we so clearly demonstrate the secondary 

 importance of a specific type ( ' Bild ') , regarded as a whole 

 as opposed to the independent factors which constitute it.' 

 The qualities, or rather their material substratum, are there- 

 fore independent of one another, and miscible to almost any 

 extent. 



Those ultimate vital particles or pangenes, which de Vries 

 substitutes for Darwin's gemmules, are therefore the bearers of 

 constituent qualities of the species. 



The fundamental idea of de Vries's whole deduction is doubt- 

 less perfectly correct. Some ten years ago, when I first began 

 to devote my attention to the problem of heredity, I fully believed 

 in the possibility of an epigenetic theory, but, as will be seen in 

 the course of this book, have long since given up this idea as 

 untenable. I too now believe that the hereditary substance is 

 composed of primary constituents, and even trust that I can prove 

 this assumption to be not only sound, but inevitable. But, at 

 the same time, I do not imagine that it suffices as an explanation 

 of the phenomena of heredity. According to de Vries, the germ- 

 substance is formed of a number of different kinds of pangenes, 

 of which as many are present as there are qualities in the species. 

 He does not consider these pangenes as arranged in any definite 

 grouping, but as freely miscible, in accordance with the assumed 



