HISTORICAL DEVELOPMENT 5 



a comparative study of certain polydemic species common to the arctic 

 islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these meth- 

 ods 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 dif- 

 ferences of structure to the definite cause. In a paper that has just ap- 

 peared, E. S. Clements (1905) has applied the method of polydemic com- 

 parison to nearly a hundred species of the Rocky mountains. In this work, 

 the all-important advance has been made of determining accurately the de- 

 cisive 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 re- 

 search 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 differ- 

 ences of water and light. 



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 ex- 

 periments 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. 



