=191. Scope and advantages.= By means of experiments actually made in the field, practically every species that is capable of modification can be made to produce new forms, the origin of which can be traced in the manner already indicated. Field experiments of this sort are especially favorable to the production of ecads from adaptable species. No attempt has yet been made to apply it to mutable or variable species, but its ultimate application to these does not seem at all impossible. The chief advantage of the method of habitat cultures is seen in the great range of choice in selecting the plant for experiment, and the habitat or area in which the experiment is carried out. A polydemic species which already has one or more ecads can be extended to a number of different habitats of known value, and a complete series of ecads obtained, based either upon water-content, or light, or upon both. On the other hand, an endemic species, or one brought from a remote flora, can be placed in as many habitats as desired, and the appearance of ecads followed in each. Frequently, results of much value are obtained in a diversified habitat by growing its most plastic species in those areas which show the greatest differences in water-content or light intensity. Habitat cultures give results which are practically as perfect as those obtained from natural experiments, since the course of adaptation in no wise depends upon whether the agent by which the seed or propagule is carried into the new habitat is natural or artificial. Cultures of this kind further possess the distinct advantage of permitting more or less modification of the physical factors themselves. However, when it is desirable to have the factors under as complete control as possible, it is necessary to use the method of control cultures in the planthouse.