It was noted in the paragraph before the last that fluctuating variants were sometimes so far off the usual that they might almost be considered distinct forms or varieties. Many such changes appear to be the result of different conditions of the local environment, due to changed conditions rather than to any internal difference in the constitution of the plant itself. A familiar illustration of environmental variation may be seen in lima beans. In any considerable number of plants one often finds smaller and larger pods, either sparsely or well filled with beans. If the beans from the small-podded, few-seeded variants are planted they will produce, apparently quite indiscriminately, large and small podded progeny, just as there will result a mixed progeny if only beans from the well-filled and large-podded kind are sown. In other words the plant fluctuates about a general average which typifies the usual or mass characteristics of the species. One should not, however, regard _all_ variations of the character in lima beans, or any other plant as environmental or fluctuating ones, for some of them may be due to differences in hereditary constitution. And these could only be determined by breeding tests. Environmental variations are as frequent as the ever-changing conditions of plant growth may determine, and it is common knowledge that such diversity of the environment and the variants resulting from it are extremely frequent. A much more fruitful source of new forms of plant life results from natural cross-fertilization, which, as we saw in an earlier chapter, is the nearly universal condition in the plant world. If species and varieties can be distinguished only by factor differences, as the plant breeders no doubt correctly insist, it becomes obvious enough that we have in cross-fertilization to consider not alone the factor differences of the pistillate or female, but also of the staminate or male contribution to the union, and how these are reflected in the progeny. Our knowledge of this has practically all been based on work done on cultivated plants under control conditions and it shows some interesting developments which occur from crossing.