=7. Experimental ecology.= Properly speaking, the experimental study of ecology dates from Bonnier[1] (1890, 1895), though it is well understood that experimental adjustment of plants to certain physical factors had been the subject of investigation before this time. The chief merit of Bonnier’s work, however, lies in the fact that it was done out of doors, under natural conditions, and for these reasons it should be regarded as the real beginning of this subject. Bonnier’s experiments were made for the purpose of determining the effect of altitude. Culture plots of certain species were located in the Alps and the Pyrenees, and the results were compared with control cultures made in the lowlands about Paris. In 1894 he also made a comparative study of certain polydemic species common to the arctic islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods are fundamental to field experiment, but the results are inconclusive, inasmuch as altitude is a complex of factors. As no careful study was made of the latter, it was manifestly impossible to refer changes and differences of structure to the definite cause. In a paper that has just appeared, E. S. Clements (1905) has applied the method of polydemic comparison to nearly a hundred species of the Rocky mountains. In this work, the all-important advance has been made of determining accurately the decisive differences between the two or more habitats of the same species in terms of direct factors, water-content, humidity, and light. In his own investigations of Colorado mountain vegetation, the author has applied the method of field cultures by planting seeds of somewhat plastic species in habitats of measured value, and has thought to initiate a new line of research by applying experimental methods to the study of vegetation as an organism. In connection with this, there has also been developed a method of control experiment in the plant house under definitely measured differences of water and light.