Let us consider briefly the possible significance of these facts in the light of Mendelian teaching. First, then, it is clear that as regards most of such cases the hypothesis is not excluded that these recurring sports may be due to the fortuitous concurrence of certain scarcer hypallelomorphs, which may either have been free in the original parent varieties from which the modern standard forms were raised, or may have been freed in the crossing to which the latter owe their origin (see p. 28). This possibility raises the question whether, if we could make “_pure_ cultures” of the gametes, any variations of this nature would ever occur. This may be regarded as an unwarrantable speculation, but it is not wholly unamenable to the test of experiments. But variability, in the sense of division of gonads into heterogeneous gametes, may surely be due to causes other than crossing. This we cannot doubt. Cross-fertilization of the zygote producing those gametes is _one_ of the causes of such heterogeneity among them. We cannot suppose it to be the sole cause of this phenomenon. When Mendel asserts the purity of the germ-cells of cross-breds he cannot be understood to mean that they are _more pure_ than those of the original parental races. These must have varied in the past. The wrinkled seed arose from the round, the green from the yellow (or _vice versâ_, if preferred), and probably numerous intermediate forms from both. The variations, or as I provisionally conceive it, that differentiant division among the gametes of which variation (neglecting environment) is the visible expression, has arisen and can arise at one or more points of time, and we have no difficulty in believing it to occur now. In many cases we have clear evidence that it does. Crossing,--dare we call it asymmetrical fertilization?--is _one_ of the causes of the production of heterogeneous gametes--the result of divisions qualitatively differentiant and perhaps asymmetrical[70]. [70] The asymmetries here conceived may of course be combined in an inclusive symmetry. Till the differentiation can be optically recognized in the gametes we shall probably get no further with this part of the problem. There are other causes and we have to find them. Some years ago I wrote that consideration of the causes of variation was in my judgment premature[71]. Now that through Mendel’s work we are clearing our minds as to the fundamental nature of “gametic” variation, the time is approaching when an investigation of such causes may be not unfruitful. [71] _Materials for the Study of Variation_, 1894, p. 78. Of _variation_ as distinct from _transmission_ why does Professor Weldon take no heed? He writes (p. 244): “If Mendel’s statements were universally valid, even among Peas, the characters of the seeds in the numerous hybrid races now existing should fall into one or other of a few definite categories, which should not be connected by intermediate forms.”