Intraspecific hybrids between 4 sets of natural races of the Madiinae of the Com-positae and Potentilla of the Rosaceae are discussed. (1) The F1 hybrid between the geographically separated and morphologically distinct but ecologically similar subspp.. typica and macrocephala of Hemizonia angustifolia, [image] = 10, is completely fertile, but the F2 shows various degrees of reduction in vigor in 43% of the plants. The genes deter-mining the physiologic processes in the 2 subspp. are therefore no longer completely interchangeable, indicating a stage in evolutionary differentiation that is transitional between that of subspecies and full-fledged species. (2) A narrow endemic of the Compositae from the inner Coast Range of California was thought to be a new genus of uncertain tribal affiliation, but it was found to be only a new subsp. of the morphologically very different Layia glandulosa, [image] = 8, with which it easily crosses and gives a completely fertile F1 and a vigorous F2 that segregates for characters generally thought to indicate tribal differences. (3) Maritime and inland climatic races, or ecotypes, of L. platyglossa, [image] = 7, differ in habit and earliness. They cross easily, the F1''s are completely fertile, and the F2 and F3 remain more vigorous than the parental types. Linkage occurs between the ecologically important characters of erect habit and early flowering of the inland race versus prostrate habit and late-ness in the maritime race. Far separated colonies of the maritime race produce only maritimes when crossed, indicating identical arrangement of the determining genes. (4) About 7 altitudinal climatic races occur in Potentilla glandulosa, [image] = 7, in central California from near the coast to 11,000 feet altitude in the Sierra Nevada. A hybrid between a coastal, winter-active and an alpine, winter-dormant race segregated for this physiological characteristic and many morphological ones. All 575 F2 individuals of a hybrid between a foothill and an alpine race were cloned and grown simultaneously in gardens near sea level at Stanford, and at 4600 ft. and 10,000 ft. altitude in the Sierra Nevada. The parents differ in some 16 morphological and physiological characteristics, each detd. by several genes. Much genetic linkage was observed between the 2 kinds of characters, a situation that tends to keep the climatic races distinct at their points of contact. The linkage is not absolute, but many recombinations occur, resulting in variation surpassing that in wild populations. Offspring that reacted similarly to the parental races but differed morphologically from them were found. The equivalents of new climatic races were also in the process of development, including plants that grew most vigorously at the lowland station like the coastal race but were winter-dormant, and others that were more adaptable than any from the wild and survived vigorously at all 3 stations. The results suggest that both geographically and ecologically isolated races of a species are regulated through the interaction of many genes in a balanced system, and that each difference is detd. by a series of genes of the multiple type, each with a minor but additive effect. The type of new combinations obtained from crossings between contrasting geographic or ecologic races indicates that there is a backlog of unutilized evolutionary resources from which superior races can be synthesized capable of fitting into many new environments or succeeding over a wider range than their parents. This is of importance for the understanding of evolution and for plant breeding.