ABSTRACT
Cross-fertilization proved to be beneficial, and self-fertilization injurious. Allied species differ greatly in the means by which cross-fertilization is favoured and cross-fertilization avoided. The possibility of cross-fertilization depends mainly on the presence and number of certain insects, often of insects belonging to special groups, and on the degree to which they are attracted to the flowers of any particular species in preference to other flowers all circumstances likely to change. The advantages of cross-fertilization depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, from what we call in our ignorance spontaneous variation. The sexual elements of the plants which were intercrossed in each generation retained sufficient differentiation during several years for their offspring to be superior to the self-fertilized, but this superiority gradually and manifestly decreased, in the result between a cross with one of the intercrossed plants and with a fresh stock.
