EXPERIMENTAL EVOLUTION I53 



METHOD OF HABITAT CULTURES 



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 ex- 

 tended 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. 



192. Methods. All field experiments in evolution are based upon a 

 change of habitat. The latter is accomplished by the modification of the 

 habitat itself, or by the transfer of the species to one or more different hab- 

 itats, or to different areas of the same habitat. In both cases the choice of 

 habitats is made upon the basis of efficient differences of water-content or 

 light. Saline situations do not constitute an exception, since the chresard 

 is really the eft'cctivc stimulus. Cultures at different altitudes, which afford 

 striking results, appear to concern several factors, but in the final analysis, 

 water-content and humidity are alone found to be really formative. Cul- 

 tures may furthermore be distinguished as simple or reciprocal. Simple 

 cultures are those in which a species is transferred to one or more habitats, 

 or in which a habitat is modified in one or more ways. Reciprocal cultures 

 are possible only with polydemic species, or with endemics after ecads have 



