Heredity on the one hand and variation on the other are the two forces upon which the origin of new species or kinds of plants is based. Both of these work in rather definite ways, some of which are fairly well understood, but many of which are still among the things that scientists are striving to clear up. As the capacity to inherit characters from one generation to another reflects itself in the generally stable conditions which the plant world exhibits, while the capacity to vary is the only source of new forms, it is quite naturally the variations of plants from one generation to another which have been most studied. And the study of variation in plants is not the simple thing we might assume it to be, having in mind only the well-known fact that no two organisms are exactly alike. Wherein do they differ? Are their perhaps temporary or even quite casual differences passed on to their progeny? These and many other questions about variation make it at once the most complicated and often one of the most fruitful subjects of plant research. It is clear enough that with the bewildering variety of different plants in nature it is next to impossible not only to record accurately the amount of variation or its probability of being handed on, but least of all to arrive at any clue as to the origin of that variation. Because of this, and still more because practically no one has ever seen the actual origin of a new species in nature, for we only see the finished product, practically all our knowledge of the laws of variation has been derived from studying cultivated plants. The ease of controlling them and of recording thousands of observations of their characteristics has made the work of the plant breeder, and others who study variation in the vegetable world, much more of an exact science to-day than the mass of often interesting but usually unrelated data that crowd the pages of older botanical literature.