No. 520] 



EVOLUTION IN VIOLA 



are capable when crossed of giving rise to 64 distinct and 

 stable forms; if they differ in eight characters, of giving 

 rise to 256 stable forms ; if in ten characters, of giving rise 

 to over 1,000. 



One or two particular cases must suffice to illustrate 

 how new and stable forms are artificially produced by the 

 propagation of hybrid offspring. In September, 1905, 

 with Mr. Witmer Stone, of Philadelphia, I visited Ivy 

 Hill Cemetery, an interesting violet station, from which 

 the previous May he had sent me several anomalous forms 

 of Viola. Two of these proved to be hybrids of a species 

 since named V. Stoneana; one with V. papilionacea, and 

 the other with V. triloba. V. Stoneana has palmately 

 dissected leaves and light-yellow seeds; V. papilionacea 

 has undissected leaves, and dark-brown seeds. Among 

 the progeny of their hybrid has appeared a form with the 

 undissected leaves of V. papilionacea and the light-yellow 

 seeds of V. Stoneana. This form comes true to seed (as 

 was to be expected, since both characters presented are 

 recessive), and shows no longer any marks of hybridity 

 or infertility. Thus has appeared in the garden a stable 

 variety of V. papilionacea with light-yellow seeds. 



In the second hybrid, V. Stoneana X triloba, both par- 

 ents have dissected leaves, though of unlike pattern ; but V. 

 triloba differs from V. Stoneana in being markedly pubes- 

 cent. The first sowing of the hybrid seed gave me only 

 four plants, one quite glabrous, three quite pubescent. 

 Seeds of these were sown the following year, and from one 

 of the three pubescent plants were obtained nine off- 

 spring, all pubescent with the leaf-form of V. Stoneana. 

 So here we seem to have a pure dominant— a stable ex- 

 hybrid— a new and pubescent variety of V. Stoneana. 

 Just such a plant was once sent me by Mr. House, col- 

 lected in the vicinity of Washington. 



Leaving now the many cases of dehybridization in 

 Viola, that have arisen in a few years of garden culture, 

 let us glance at the evidence that this same process is 

 going on in a state of nature, and has probably been going 

 on for centuries. 



