528 THEORETICAL ORIGIN OF EXDOGENS FROM EXOGENS. 



alterations of structure in amphibious plants, according as they 

 live in water or on land. I think these coincidences are sufficient 

 to prove that endogens have, in the first place, descended from 

 very early types of exogens which preceded them; and that, 

 secondly, the more immediate cause of their origin was an aquatic 

 habit of life assumed by certain primitive exogenous plants. 



To state the process of adaptation in as few words as possible, 

 one might say that it was effected by means of the joint action 

 of the external influences or forces of the environment and the 

 internal responsive power of living protoplasm ; so that when a 

 terrestrial and aerial plant is grown in water, or a water-plant is 

 transferred to land, the direct actiou of the new environment 

 compels the plant (provided it can survive the change) to so 

 alter its structure, i. e. of its histological elements — which in 

 turn, of course, cause changes in the morphological — that the 

 plant now becomes at once adapted to the new medium. The 

 change from land to water brings on what may be therefore called 

 11 adaptive degenerations " in every part of the plant. Con- 

 versely, a transference from water to land induces a tendeucy to 

 restore the lost features, resulting, it may be, in a greatly enhanced 

 and vigorous growth. 



Whether there were several, few, or only one originally aquatic 

 ancestor, though probability would suggest several, I would 

 not even venture to guess ; because it is impossible to trace 

 affinities between the flowers of existing endogens and those of 

 any existing orders of exogens ; but that the theory I have 

 advanced is at least a plausible one seems to me to be sufficiently 

 established. 



