=8. Ecology of the habitat.= Since the time of Humboldt, there have been desultory attempts to determine the physical factors of habitats with some degree of accuracy. The first real achievement in this line was in the measurement of light values by Wiesner in 1896. In 1898 the writer first began to study the structure of habitats by the determination of water-content, light, humidity, temperature, wind, etc., by means of instruments. These methods were used by one of his pupils, Thornber (1901), in the study of a particular formation, and by another, Hedgcock (1902), in a critical investigation of water-content. Two years later, similar methods of measuring physical factors were put into operation in connection with experimental evolution under control in the plant house. E. S. Clements (1905), as already indicated, has made the use of factor instruments the foundation of a detailed study of polydemic species, i. e., those which grow in two or more habitats, and which are, indeed, the most perfect of all experiments in the production of new forms. In a volume in preparation upon the mountain vegetation of Colorado, the writer has brought the use of physical factor instruments to a logical conclusion, and has made the study of the habitat the basis of the whole work. Out of this investigation has come a new concept of vegetation (Clements 1904), namely, that it is to be regarded as a complex organism with structures and with functions susceptible of exact methods of study. =9. The evidence from historical development.= This extremely brief resume of what has been accomplished in the several lines of ecological research makes evident the almost complete absence of correlation and of system. The whole field not merely lacks system, but it also demands a much keener perception of the relative value of the different tendencies already developed. It is inevitable from the great number of tyros, and of dilettante students of ecology in comparison with the few specialists, that the surface of the field should have received all of the attention. It is, however, both unfortunate and unscientific that great lines of development should be entirely unknown to all but a few. There is no other department of botany in which the superficial study of more than half a century ago still prevails to the exclusion of better methods, many of which have been known for a decade or more. It is clear, then, that the imperative need of ecology is the proper coordination of its various points of view, and the working out of a definite system which will make possible a ready recognition of that which is fundamental and of that which is merely collateral. The historical development, as is well understood, can furnish but a slight clue to this. It is a fact of common knowledge that the first development of any subject is general, and usually superficial also. True values come out clearly only after the whole field has been surveyed.