18 BOTANY. [CHAP. i. 



and reproductive portions a group of plants eventually 

 evolve, which if not traced through all the sequence of 

 changes to the parent form, would very probably never 

 be suspected of having evolved from the latter. It is 

 not to be inferred from the above that all existing 

 groups, as at present understood, have originated by 

 variation ; another factor of equal importance degenera- 

 tion or the arrest and cutting down of structures 

 already highly evolved, has in all probability produced 

 as many or even more groups, distinct as such at the 

 present day, than the one previously explained. The 

 results of degeneration are as a rule more obvious in the 

 reproductive than in the vegetative portions of plants, 

 and illustrated by the cutting down or arrest of the corolla 

 or showy portion of the flower that was of functional value 

 when the flower was fertilized by insect agency, but has 

 disappeared as the plant adopted the method of self- 

 fertilization, or became so modified that the pollen or 

 fertilizing substance is conveyed by wind. The grasses 

 afford an illustration of a group of plants that are wind- 

 fertilized at the present day. 



Returning for a moment to the division of labour 

 necessitated by the change from an aquatic to an aerial 

 habitat, it was soon realized that desiccation or loss of 

 water, an indispensable substance in connection with 

 active life, should be guarded against ; various primitive 

 methods, each more or less successful in its way, were 

 tried by different groups, more especially the massing 

 together of a large number of individuals, whereby the 

 required amount of moisture was better retained than in 

 the case of the same number of isolated individuals ; but 

 notwithstanding the various efforts on the part of uni- 



