Defective inheritance-ratios in Bursa hybrids. 
By Dr. George Harrison Shull, 
Station for Experimental Evolution, Cold Spring Harbor, Long Island^ 
New York. 
Several years ago (1905 — 1907) I was making extensive 
cultures of Bursa ( Capsella) hursa-joastoris^ of Bursa Heegeri, and 
of hybrids between these two species. Bursa Heegeri is generally 
conceded to have originated from B. bursa-pastoris by a recent 
mutation and Las been found in nature only once in a Situation 
which would vrarrant the belief that it had not been derived 
from a near - by experimental culture. Although a niimber of 
specimens were found by Professor PI e e g e r at the original locality 
on the market-place at Landau, Germany, in 1897, so far as is 
now known these represented a single elementary form or biotype. 
Bursa bursa-pastoris , on the other band, is of almost world-wide 
distribution, and presents an unknown number, but certainly a 
large number, of distinct biotypes. 
In my cultures of the latter species I found four forms of 
rosette which were related to one another as the four terms of 
a Mendelian di-hybrid. These four biotypes were named and 
described as foUows : ^) 
Type (a). Bursa bursa-pastoris Tieteris has the leaves divided 
to the mid-rib, the terminal lobe being usually separated from 
the nearest lateral lobes by clean, deep incisions. The lateral 
lobes consist essentially of two features, an elongated proximal 
portion, the „primary lobe", and a more or less rounded or 
1) „Results of Crossing Bursa bursa-pastoris and Bursa Heegeri"", 
6 pp. Proceedings Seventh International Zoological Congress, Boston Meeting, 
August 19—24 1907. „Advanced reprint" issued in 1910. 
„Bursa bursa-pastoris and Bursa Heegeri: Biotypes aud hybrids''. 
07 pp., 23 text-figs., 4 pls., Pabl. No. 112, Carnegie Institution of 
Washington, 1909. 
