94 The Outline of Science 



Cross-fertilisation is much surer by insects than by the wind, and 

 cross-fertilisation is more advantageous than self-fertilisation be- 

 cause it promotes both fertility and plasticity. It was probably 

 in this period that coloured flowers attractive to insect-visitors 

 began to justify themselves as beauty became useful, and be- 

 gan to relieve the monotonous green of the horsetail and club- 

 moss forests, which covered great tracts of the earth for millions 

 of years. In the Carboniferous forests there were also land- 

 snails, representing one of the minor invasions of the dry land, 

 tending on the whole to check vegetation. They, too, were prob- 

 ably preyed upon by the Amphibians, some of which attained a 

 large size. Each age has had its giants, and those of the Carboni- 

 ferous were Amphibians called Labyrinthodonts, some of which 

 were almost as big as donkeys. It need hardly be said that it was 

 in this period that most of the Coal-measures were laid down by 

 the immense accumulation of the spores and debris of the club- 

 moss forests. Ages afterwards, it was given to man to tap this 

 great source of energy traceable back to the sunshine of mil- 

 lions of years ago. Even then it was true that no plant or animal 

 lives or dies to itself ! 



The Acquisitions of Amphibians. 



As Amphibians had their Golden Age in the Carboniferous 

 period we may fitly use this opportunity of indicating the ad- 

 vances in evolution which the emergence of Amphibians implied. 

 (1) In the first place the passage from water to dry land was 

 the beginning of a higher and more promiseful life, taxed no 

 doubt by increased difficulties. The natural question rises why 

 animals should have migrated from water to dry land at all when 

 great difficulties were involved in the transition. The answers 

 must be: (a) that local drying up of water-basins or elevations 

 of the land surface often made the old haunts untenable; (b) that 

 there may have been great congestion and competition in the old 

 quarters; and (c) that there has been an undeniable endeavour 



