2 VII. INTERNATIONAL ZOOLOGICAL CONGRESS 



are other characters of the stem and inflorescence in addition to the 

 capsule character noted by Solms-Laubach, which serve to distinguish 

 B. heegeri from its supposed parent, but on the other hand he found 

 that in both species there occur frequent instances of abnormal pis- 

 tillate but sterile flowers in the lower portion of the flower-stem, and 

 he took this fact together with the occurrence of capsules occasionally 

 simulating those of B. bursa-pastoris, formed when B. heegeri is at- 

 tacked by Cystopus and Peronospora, as additional proofs of the 

 correctness of the view that B. heegeri is a derivative from B. bursa- 

 pastoris. Of the manner of its derivation from B. bursa-pastoris 

 nothing is known, of course, but Potonie 1 suggests that its origination 

 was a pathological phenomenon with atavistic significance; in other 

 words, that it is a reversion induced by some pathological condition. 



The prominent part which mutation may have taken in the produc- 

 tion of new species makes it of great interest to know in each case 

 just what will be the result both of self-fertilization and of crossing 

 the mutant with the parental form, for its behavior in these two 

 cases is the important factor in determining its power of mainte- 

 nance at the time of its origin. The result of self-fertilization was 

 investigated by Solms-Laubach before the publication of his original 

 account, and it was shown that the characters that differentiate B. 

 heegeri from B. bursa-pastoris are fully heritable. As the Bursas 

 normally self-fertilize to a predominant extent, the possession of a 

 vigorous constitution would seem to be all that is necessary for 

 successful maintenance. 



I have now tested the question as to what will happen when Bursa 

 heegeri and B. bursa-pastoris intercross. In order to appreciate the 

 result fully it will be necessary to recount briefly some recently dis- 

 covered facts regarding the composition of the species B. bursa- 

 pastoris. This species is not of unit value, but is a composite group of 

 elementary species as has been shown by Lotsy, 2 Shull, 3 and Almquist. 4 

 How many of these elementary species there may be cannot be 

 stated at this time, but Almquist has described and named over 70. 

 In my own cultures I have observed that some of the differentiating 

 characters used by Almquist disappear upon continued breeding, 

 and I think it very probable that not all of the 70 elementary species 

 described by him will stand the test of longer culture under more 

 carefully controlled conditions, but it seems evident that the number 

 of elementary forms may be considerable. I have studied in their 

 hybrid combinations four of these elementary species, and have 



1 Potonie, H., Capsella Heegeri eine pathologische Erscheinung mit ata- 

 vistischen Momenten? Naturwis. Wochenschr. 21 (N. S. 5), pp. 788-791. 1906. 



2 Lotsy, J. P., Vorlesungen iiber Deszendenztheorien, mit besonderer Be- 

 riicksichtigung _der botanischen Seite der Frage. ErsterTeil. pp. 179-181. 1906. 



3 Shull, G. H., Elementary species and hybrids of Bursa. Science, N. S. 

 25, pp. 590-591. 1907. 



4 Almquist, E. Studien iiber die Capsella bursa-pastoris L. Acta Horti Ber- 

 giana. 4, No. 6, 1907. 



