In order to look closer into the real mechanism of this form of variability, and of this constant tendency to occasional reversions, it will be best to limit ourselves first to a single case, and to try to gather all the evidence, which can be obtained by an examination of the hereditary relations of its sundry constituents. [314] This may best be done by determining the degree of inheritance for the various constituents of the race during a series of years. It is only necessary to apply the two precautions of excluding all cross-fertilization, and of gathering the seeds of each individual separately. We do not need to ascertain whether the variety as such is permanent; this is already clear from the simple fact of its antiquity in so many cases. We wish to learn what part each individual, or each group of individuals with similar characters, play in the common line of inheritance. In other words, we must build up a genealogical tree, embracing several generations and a complete set of the single cases occurring within the variety, in order to allow of its being considered as a part of the genealogy of the whole. It should convey to us an idea of the hereditary relations during the life-time of the variety. It is manifest that the construction of such a genealogical tree requires a number of separate experiments. These should be extended over a series of years. Each should include a number of individuals large enough to allow the determination of the proportion of the different types among the offspring of a single plant. A species which is easily fertilized by its own pollen, and which bears capsules with [315] large quantities of seeds, obviously affords the best opportunities. As such, I have chosen the common snapdragon of the gardens, _Antirrhinum majus_. It has many striped varieties, some tall, others of middle height, or of dwarfed stature. In some the ground-color of the flowers is yellow, in others it is white, the yellow disappearing, with the exception of a large mark in the throat. On these ground-colors the red pigment is seen lying in streaks of pure carmine, with white intervals where the yellow fails, but combined with yellow to make a fiery red, and with yellow intervals when that color is present. This yellow color is quite constant and does not vary in any marked degree, notwithstanding the fact that it seems to make narrower and broader stripes, according to the parts of the corolla left free by the red pigment. But it is easily seen that this appearance is only a fallacious one. The variety of snapdragon chosen was of medium height and with the yellow ground-color, and is known by horticulturists as _A. majus luteum rubro-striatum_. As the yellow tinge showed itself to be invariable; I may limit my description to the red stripes.