150 THE PLANT 



graphic record of the course of variation. When the latter is made evident 

 in structural feature also, histological slides are an invaluable part of the 

 record. 



Polydemic species are by far the best and most frequent of all natural 

 experiments. In addition to plants that are strictly polydemic, i. e., grow 

 in two or more distinct habitats, there are a large number which occur in 

 physically different parts of the same habitat. The recognition of polydemics 

 is the simplest of tasks. As a rule, it requires merely a careful examination 

 of contiguous formations in order to ascertain the species common to two 

 or more of them. The latter are naturally most abundant along the eco- 

 tones between the habitats, and, as a result, transition areas and mixed 

 formations are almost inexhaustible sources of ecads. Many adaptable 

 species are found throughout several formations, however, and such are 

 experiments of the greatest possible value. Not infrequently species of tht 

 manuals are seen to be ecads, in spite of their systematic treatment, and to 

 constitute natural experiments that can be readily followed. Finally, it 

 must be kept in mind that some polydemics are stable, and do not give rise 

 to ecads by structural adaptation. They not only constitute extremely in- 

 teresting experiments in themselves, but they should also be very carefully 

 followed year by year, since it seems probable that the responses are merely 

 latent, and that they will appear suddenly in the form of mutants. In 

 natural experiments it is sometimes difficult to distinguish which form is the 

 ecad and which the original form of the species. As a rule, however, this 

 point can be determined by the relative abundance and the distribution, but 

 m cases of serious doubt, it is necessary to appeal to experimental cultures. 



Although habitats differ more or less with respect to all their factors, the 

 study of polydemics needs to take into account only the direct factors, 

 water-ccntent, humidity, and light. Humidity as a highly variable factor 

 plays a secondary part, and in consequence the search for ecads may be 

 entirely confined to those habitats that show efficient differences in the 

 amount of water-content or of light. Temperature, wind, etc., do not pro- 

 duce ecads, and may be ignored, except in so far as they affect the direct 

 factors. Complexes of factors, such as altitude, slope, and exposure, are 

 likewise effective only through the action of the component simple factors 

 upon water and light. The influence of biotic factors is so remote as to 

 be negligible, especially in view of the fact that ecads are necessarily 

 favorable adaptations, and are in consequence little subject to selective 

 agencies. The essential lest of a habitat is the production of a distinguish- 

 able ecad, but a knowledge of the water-content and light values of the 

 habitats under examination is a material aid, since a minute search of each 

 formation is necessary to reveal all the ecads. It is evident that habitats or 



