286 Discontinuity [CH. 



the factor in question remains unpaired in the hybrid. The 

 latter description, "unisexual," is applied to characters which 

 breed true in the cross-bred. This suggestion is the out- 

 come of an attempt to incorporate the facts of Mendelian 

 inheritance with the conclusions previously drawn from the 

 mutations studied in Oenothera. But as knowledge of 

 Mendelian cases has increased, the applicability of what is 

 here spoken of as the "presence and absence" hypothesis 

 becomes more and more clear. We now, in fact, feel fairly 

 sure that a heterozygote is properly represented as one 

 which contains an unpaired factor. Hence the doubt may 

 be expressed whether if de Vries' terminology is to be 

 maintained, its application should not be reversed. 



2. Discontinuity in Variation. 



When some years ago I published a collection of facts 

 illustrating the phenomenon of Variation in animals*, I 

 pointed out that variation is frequently definite, or Discon- 

 tinuous. That conclusion is one which cannot fail to strike 

 an observer who makes a study of this part of physiology. 

 Inasmuch as the discontinuity of variation is manifested 

 again and again in respect of exactly those differences 

 which we are accustomed to recognize as distinguishing 

 specific forms from each other, the further conclusion 

 followed that the diversity of species may be regarded as 

 having come about very largely by the occurrence of these 

 discontinuous variations. 



The materials then put forward related almost entirely 

 to a restricted group of phenomena in animals, those which 

 are known as meristic, exemplifying the processes of change 

 in the number of parts and the relation of repeated parts 

 to each other. Had the field of inquiry been widened by 

 the inclusion of variations in other characteristics of animals, 

 or in those of plantsf, a body of evidence more clearly 

 demonstrating the truth of this thesis could have been 

 presented. There were, however, reasons which led to the 



* Materials for the Study of Variation, 1894. 



t A useful collection of facts of this nature in plants has been published 

 by Korschinsky, under the title "Heterogenesis und Evolution," Flora, 89, 

 1901. 



