96 PLANT-LIFE ON LAND [CH. 



seems clear that in the Higher Flowering plants, 

 animal agency was early available, and was soon 

 made use of. There is ample evidence of insect 

 life from the Secondary Rocks, and it was in that 

 period that the outburst of evolution of Flowering 

 plants took place. Though many of the simpler 

 types employ the agency of the wind or of water, 

 there is good reason to believe that the more com- 

 plex flower structures were evolved correlative to 

 and together with their insect visitors, a parallelism 

 as probable as it is interesting. There may doubtless 

 have been cases of transfer from the one agency to 

 the other, as for instance in the Rue (Thalictrum\ 

 which has become wind-pollinated in a family 

 (Ranunculaceae) as a rule insect-pollinated ; and 

 the converse is also possible. It is not our object 

 here to define the actual degree of dependence for 

 pollination upon one or the other of these two 

 important agencies, but rather to indicate that, 

 whatever the detail, this principle rules in Seed- 

 plants : that in very many cases to secure pollina- 

 tion at all, and in all cases to secure the advantages 

 of intercrossing, some agency outside the plant itself 

 is a necessity ; and that this necessity arises primarily 

 from the immobility imposed by the rooted habit. 



The fixed position is then the prime factor leading 

 to all those wonderful adaptations of the flower which 

 have pollination as their end. It may also be held as 



