These physiologic units, which in the end must be the basis for the distinction of the systematic units, may best be designated by the term of "unit-characters." Their internal nature is as yet unknown to us, and we will not now look into the theories, which have been propounded as to the probable material basis underlying them. For our present purpose the empirical evidence of the general occurrence of [250] sharp limits between nearly related characters must suffice. As Bateson has put it, species are discontinuous, and we must assume that their characters are discontinuous also. Moreover there is as yet no reason for trying to make a complete analysis of all the characters of a plant. No doubt, if attained, such an analysis would give us a deep insight into the real internal construction of the intricate properties of organisms in general. But taxonomic studies in this direction are only in their infancy and do not give us the material required for such an analysis. Quite on the contrary, they compel us to confine our study to the most recently acquired, or youngest characters, which constitute the differentiating marks between nearly allied forms. Obviously this is especially the case in the realm of the hybrids, since only nearly related forms are able to give hybrid offspring. In dealing with this subject we must leave aside all questions concerning more remote relationships. It is not my purpose to treat of the doctrine of hybridization at any length. Experience is so rapidly increasing both in a practical and in a purely scientific direction that it would take an entire volume to give only a brief survey of the facts and of all the proposed theories. [251] For our present purposes we are to deal with hybrids only in so far as they afford the means of a still better distinction between elementary species and varieties. I will try to show that these two contrasting groups behave in quite a different manner, when subjected to crossing experiments, and that the hope is justified that some day crosses may become the means of deciding in any given instance, what is to be called a species, and what a variety, on physiologic grounds. It is readily granted that the labor required for such experiments, is perhaps too great for the results to be attained, but then it may be possible to deduce rules from a small series of experiments, which may lead us to a decision in wider ranges of cases. To reach such a point of view it is necessary to compare the evidence given by hybrids, with the conclusions already attained by the comparison of the differentiating characteristics of allied forms. On this ground we first have to inquire what may be expected respecting the internal nature and the outcome of the process of crossing in the various cases cited in our former discussion.