^^ Rhodora [January 



islands as consisting of species distinct from hut allied to those of the 

 neighboring mainland; free intercourse over a connecting isthmus 

 would have largely prevented this divergence into two sets of species.^ 

 In many genera we know that certain species renuiin distinct only 

 because each has its separate range; if through the agency of man 

 they are brought together, they become more or less confluent through 

 interbreeding. — Some years since I was interested in the study of 

 Aquilegia; I got together as many speci(\s as possible of both the Old 

 World and the New, and became quite familiar with the various types. 

 Afterward, when they had been growing for several years without 

 cultivation on the north side of an arbor-vitae hedge, my attention 

 was attracted by the appearance of forms that I had never seen before. 

 I found that at least five different hybrids had a{)peared spontaneously. 

 Similar instances are by no means rare. 



An analogous cause has disturbed the relations to each other of our 

 species of common blue violets. Two or three centuries ago the north- 

 eastern United States was entirely covered with forests. ^Fhe clearing 

 away of these forests by man has profoundly modified the conditions 

 of plant life. With the exception of T. palmafa, our blue stemless 

 violets are rarely found, and certainly do not thrive, in the primeval 

 forest; they are rather jilants of o])en groves, of fence-rows, and moist 

 meadows. When the whole country was densely wooded, their growth 

 must have been greatly restricted; but when the trees were removed 

 they had a chance to 'run and be glorified'; their range was vastly 

 extended; the lunnber of individuals increased perhaps a thousand- 

 fold; species before isolated became cohabitant, and had opportunities 

 to hybridize.'^ 



How far this movement of involution will extend, it is idle to specu- 

 late. But so far as regards the survival of our species of Viola we 

 need give ourselves no concern; for the vast majority of its seeds are 

 produced from the self-fertilized flowers, and must therefore be free 

 from admixttire witii other species. However readily the ])etaliferous 

 flowers nuiy produce hybrids, these hybrids must always be greatly 

 outnumbered by the legitimate offspring. 



MrnDLioHUKv College. 



• The Origin of Species, 6th ed., ii. 177-182. 



2 It would he interesting to follow out this line of thought in its hearing on otlier group.s 

 of species, such as Crataegu.s and Ruhiis. There are not wanting grounds for the suspi- 

 cion that they too, since the forests were cleared, have enormousl.v multiplied, and have 

 been behaving badly, — producing liybrids and Mendelian crosses, — sporting wantonly, 

 — at the expense of the makers of species. 



