952 ECOLOGY 



factors. Sometimes reference is made instead to releasing factors, the 

 structure in question beipg regarded as potentially present, though its 

 manifestation is inhibited until the proper factor enters in place of or 

 in addition to the inhibiting factor. In the case of reaction structures 

 it is believed that the form assumed, as well as the time and the place of 

 appearance, is due to external agents, wherefore the latter are called 

 formative factors. 



While there is no doubt of the reality of reaction structures, because 

 they are so readily capable of experimental production, there are many 

 investigators who disbelieve in the reality of congenital structures. An 

 alternative hypothesis is that all plant structures are or have been plastic. 

 The so-called rigid or congenital structures may in some ancestral form 

 have been as plastic as are the reaction structures of to-day ; in this event 

 the fixation of reaction structures has resulted in structures that now are 

 congenital. It is equally possible that the supposedly rigid congenital 

 structures really are plastic, but to an imperceptible degree, as compared 

 with the leaf plasticity of amphibious plants; if this is true, all plant 

 structures are plastic, some obviously and rapidly, and others so slightly 

 or slowly that only experiments of long duration can reveal plasticity. 

 While in the present state of imperfect knowledge, it is convenient and 

 not necessarily incorrect to contrast reaction structures and congenital 

 structures, it is possibly more correct to subdivide plant structures into 

 those that certainly are plastic and those that apparently are rigid. 



The survival of advantageous structures through natural selection. — 

 While the origin of structures through reaction or through mutation can- 

 not account for the present preponderance of advantageous structures 

 and advantageous behavior, the theory of natural selection is in this re- 

 spect as satisfactory as it has proven unsatisfactory from the standpoint 

 of causation.' Of the species with new congenital structures, only 

 those are likely to survive that happen to be suited for existence in the 

 habitat in which the new structures develop, or that are able to migrate 

 to a suitable habitat ; a mesophyte that happens to originate in a desert 

 or a plant suited to warm climates that happens to originate in a cold 

 region cannot survive. Of the more plastic species only those are likely 



' Against natural selection as a causative theory there may be virged: (i) the existence 

 in many species of a capacity for advantageous regeneration, though the opportunity for 

 such regeneration rarely if ever occurs in nature; (2) the existence of complicated struc- 

 tures (such as stinging hairs, digestive glands, and extra-floral nectaries) whose value to the 

 plants possessing them is slight; and (3) the existence of "overadaptation," as in the 

 flowers of orchids and in the seeds of certain xerophytes. 



