ON TWIN HYBRIDS. 475 



the fall, are liable to display their differential marks in a most striking 

 manner. 



The twins are usually produced from the same cross in about 

 equal numbers. On account of their highly diminished fertility it is 

 difficult to get large cultures of their progeny, for even when the 

 pods are large and contain an apparently full supply of seed, the 

 germinating percentage is often very small. The following figures, 

 however, will suffice to state the fact. 



Table I 

 0. laeta and 0. velutina in first generation 



Number of Percentage Percentage 

 Mother Father progeny of laeta of velutina 



0. biennis x 0. brevistylis 85 47 53 



0. biennis x 0. rubrinervis 50 42 58 



0. b. cruciata x 0. brevistylis 91 48 52 



0. muricata x 0. Lamar ckiana .... 58 61 39 



0. muricata x 0. brevistylis 120 59 41 



0. muricata x 0. nanella 59 58 42 



Average 52 48 



In this experiment the parents have been those described above, 

 and the crosses have been made by myself in the summer of 1905. 

 The countings have been made during the flowering-season of 1907, 

 only a small part of each of the cultures having remained rosettes; 

 but the distinguishing marks in these were as evident in the fall, 

 as those of the flowering plants. Some crosses, tried in previous 

 years, had given the same results and prepared the method for an 

 exact counting. 



In the young plants, before the sending-up of the stems, it is of 

 course easy to count far larger numbers of plants, but the influence 

 of transgressive fluctuability is somewhat greater. I counted for 

 0. biennis x brevistylis 287, for 0. biennis x Lamarckiana 347, and 

 for 0. biennis cruciata x brevistylis 208 seedlings, and found 55 per 

 cent., 60 per cent., and 64 per cent., or on an average 60 per cent, 

 specimens of 0. velutina, the remaining 40 per cent, being 0. laeta. 

 The figures, although from the cause given not as exact as those 

 given above, evidently confirm the result. 



Tried in the second generation, from artificially and purely self- 

 pollinated seed, each of the twins yields a uniform progeny, with 

 exactly the same characters as its parent. Therefore they may be 

 considered as constant hybrid races. 1 made the majority of the 



