In 1978, again with Axelrod, they published on the origin and complexity of Californian flora. They reviewed that the flora of California consisted of "northern, temperate elements and xeric, southern elements, and is characterized by a high degree of endemism." They proposed that the reasons for the large number of species in the state as well as the endemics is due to the favorable climate that has prevailed in California for most of the Tertiary, as well as the recent elevation of the Sierra Nevada and other ranges, together with: "The concomitant development of a cold off-shore current which ultimately resulted in the development of a mediterranean, summer-dry climate during the past million years...The endemics of California are a mixture of relicts and newly produced species...and it is the latter that have contributed most to the size of the flora and to the high proportion of endemism in it."