THE MUTATION THEORY 351 



the lamarckiana, by the vigorous growth of the style, extends the calyx 

 and renders the flower-bud thinner and more slender. Those of the 

 brevistylis are therefore broader and more swollen. It is quite easy 

 to distinguish the individuals by this striking character alone, although 

 it differs from the parent in other particulars. 



The leaves of the O. brevistylis are more rounded at the tip, but 

 the difference is only pronounced at times, slightly in the adult 

 rosettes, but more clearly on the growing summits of the stems and 

 branches. By this character the plants may be discerned among the 

 others some weeks before the flowers begin to show themselves. 



But the character by which the plants may be most easily recog- 

 nized from a distance in the field is the failure of the fruits. They 

 were found nearly every year in varying, but always small numbers. 



Leaving the short-styled primrose, we come now to the last of 

 our group of retrograde varieties. This is the 0. nannella, or the 

 dwarf, and is a most attractive little plant. It is very short of stature, 

 reaching often a height of only 20-30 cm., or less than one-fourth of 

 that of the parent. It commences flowering at a height of 10-15 cm > 

 while the parent-form often measures nearly a meter at this stage of 

 its development. Being so very dwarfed the large flowers are all the 

 more striking. They are hardly inferior to those of the lamarckiana, 

 and agree with them in structure. When they fade away the spike 

 is rapidly lengthened, and often becomes much longer than the lower 

 or vegetative part of the stem. 



The dwarfs are one of the most common mutations in my garden, 

 and were observed in the native locality and also grown from seeds 

 saved there. Once produced they are absolutely constant. I have 

 tried many thousands of seeds from various dwarf mutants, and never 

 observed any trace of reversion to the lamarckiana type. I have also 

 cultivated them in successive generations with the same result. In a 

 former lecture we have seen that contrary to the general run of 

 horticultural belief, varieties are as constant as the best species, if 

 kept free from hybrid admixtures. This is a general rule, and the ex- 

 ceptions, or cases of atavism, are extremely rare. In this respeci it is of 

 great interest to observe that this constancy is not an acquired quality, 

 but is to be considered as innate, because it is already fully developed 

 at the very moment when the original mutation takes place. 



From its first leaves to the rosette period, and through this to the 

 lengthening of the stem, the dwarfs are easily distinguished from any 

 other of their congeners. The most remarkable feature is the shape 



