EXPERIMENTAL EVOLUTION 153 
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 effective 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 
