8 BURSA BURSA-PASTOSIS AND BURSA HEEGERI : 



be expected to produce a constant intermediate progeny, nor one which 

 would show the two parental conditions in an expected Mendelian ratio. 

 Occasionally in the course of pedigree-cultures some slight difference in 

 treatment will produce a modification in some progeny, which may lead to 

 doubt as to its classification, and perhaps even after several generations 

 this doubt will not be wholly removed. In such cases hybridization is 

 almost certain to give decisive results, as there often appears to be a more 

 characteristic development of morphological features in the hybrids than in 

 the pure-bred strains. This may be due to the fact that the allelomorphs 

 are brought into different relations with other allelomorphs in the recom- 

 binations which take place in the second and later generations, thus elimi- 

 nating any inhibiting or modifying influence which may have resulted from 

 the constant presence of some particular allelomorph or combination of 

 allelomorphs, or of some permanent condition of the cytoplasm, in the pure 

 strain. This elimination of modifying factors would allow a more accurate 

 definition of the unit-characters involved. Or, on the other hand, the better 

 development of characters in the hybrids may be due to the fact, long 

 recognized, that heterozygosis produces a stimulation which increases 

 vegetative vigor, and this should have the same effect as good cultural 

 conditions in bringing all characters to their full typical development. 



MATERIAL AND METHODS. 



For the beginning of the evil tu res, seeds were secured from a number of 

 more or less strikingly different individuals growing in the vicinity of Cold 

 Spring Harbor, Long Island; Chicago, Illinois; New Carlisle, Ohio; Man- 

 hattan, Kansas; and Tucson, Arizona. Leaves of each of the plants chosen 

 as parents for the cultures were carefully preserved, in order that the con- 

 ditions of the offspring might be compared with those of the parent and, 

 in each pedigreed family raised since, care has been taken to preserve in 

 the herbarium samples of every variation of sufficient magnitude to strike 

 the eye. 



JFrom 20 lots of seeds of Bursa burs a-pas tori s brought in from nature 

 and 1 lot of B. heegeri received by Dr. MacDougal from Professor Solms- 

 Laubach, I have now had under observation about 200 pedigreed families, 

 including something over 26,900 individuals of Bursa bursa-pastoris, 2 small 

 families of Bursa heegeri, and 5 families representing reciprocal crosses 

 between these two species and involving over 2,500 individuals. 



All of my cultures have been carried on under glass, the first 7 months 

 in the sky-lighted room of the laboratory at the Station for Experimental 

 Evolution, and since that time, i. e., after December 7, 1905, in the glass 

 propagating-house at the same place. It was found that the light was too 

 dim in the former room to bring out the characteristic features of the plants 

 and to keep them in a state of vigorous health. The leaves became more 



