52 BUR.SA BURSA-PASTORIS AND BURSA HEEGERI : 



DISCUSSION OF RESULTS. 



The occurrence of elementary species within the limits of recognized 

 systematic or Linnean species is undoubtedly very g-eneral, as De Vries 

 has maintained, and much of the ordinary conception as to the variability 

 of certain species is attributable to this fact. Thus in the specific case with 

 which we are dealing here, Biirsa is recognized by all taxonomists as 

 exceedingly variable, but each single biotype of Bursa is much less vari- 

 able than the Linnean species taken as a whole, for when grown under 

 favorable conditions there are certain characteristics which are found in 

 every individual of the given biotype which are not present in the members 

 of other biotypes. 



All the apparent evidence for the permanent change of species through 

 selection finds a ready explanation on the assumption that the selection 

 has merely eliminated certain biotypes from the original mixture with 

 which the selection started, thus leaving the theory that fluctuations are 

 inherited or are capable of leading either directly or indirectly to the modi- 

 fication of any biotype wholly unsupported. This is not to say, of course, 

 that such modification is impossible or that it does not take place, but merely 

 that such a proposition must rest upon experience still to be gained. 



There is some variation among the members of the single biotype. This 

 variation is of the fluctuating kind by definition. The most usual varia- 

 tions of this kind are those which result from crowding, shading, poor soil, 

 drought, the attacks of insects, or other conditions which decrease the vigor 

 of the plants, the effect being to arrest differentiation in more or less juve- 

 nile stages of development. This feature has been very troublesome at 

 times in my cultures, since it is impossible under such circumstances to 

 determine by inspection to which biotype a given specimen belongs. The 

 breeding-test is the only method by which such a determination can be 

 made and when many specimens have their distinguishing characteristics 

 rendered latent in this way the labor, time, and patience required for com- 

 plete classification become unduly increased. 



This suppression of characteristics through fluctuation also stands in the 

 way of the classification of specimens observed in nature in various habi- 

 tats where they have grown under different conditions which are in a large 

 degree unknown, and on this account there can be no question as to the 

 advisability of retaining the Linnean names in practical taxonomy for the 

 designation of such complex groups of biotypes. Workers in all other 

 botanical fields must bear in mind, however, that conclusions reached with 

 one biotype may not hold in some other biotype of the same Linnean species. 



The demonstration of the elementary character of these biotypes of Bursa 

 is made complete by the fact that they Mendelize on being crossed, for such 



