In the following years, most biologists came to accept the Mendelian-chromosome theory, which was independently proposed by Walter Sutton and Theodor Boveri in 1902/1903, and elaborated and expanded by Morgan and his students. Garland Allen characterized the post-1915 period as one of normal science, in which "The activities of 'geneticists' were aimed at further elucidation of the details and implications of the Mendelian-chromosome theory developed between 1910 and 1915." But, the details of the increasingly complex theory, as well as the concept of the gene and its physical nature, were still controversial. Critics such as W. E. Castle pointed to contrary results in other organisms, suggesting that genes interact with each other, while Richard Goldschmidt and others thought there was no compelling reason to view genes as discrete units residing on chromosomes.