104 PROBLEMS OF EVOLUTION 



conditions that obtain in sandy deserts, but one may be more 

 protectively coloured than another and so may be better able to 

 escape birds of prey. 



Evolution For every species the environment consists mainly of other 

 vironment s P ec i es with which it is brought into contact. So that fwo evolu- 

 tions have proceeded side by side, that of the environment and that of the 

 species. The countless succession of steps by which any complex 

 animal has reached its present stage would be, had not sexual 

 selection come in as a disturbing factor, equal in number to the 

 changes of environment to which each of those steps has been in 

 its turn an adaptation. 



Race be- The race between competing species is from the nature of the 

 competing case a neck-and-neck one, a favourable variation that appears in 

 species one must be matched by a corresponding advance in those that 

 are affected by it. If birds of prey grow keener of sight and 

 stronger on the wing, the species that they hunt must improve 

 their means of escape ; their quickness on the wing or their pro- 

 tective plumage. If camels devour thorns and leaves of nauseat- 

 ing flavour with relish and voracity, then the plants that trust to 

 such means of defence must become still more thorny or nause- 

 ous. When men take to drinking methylated spirit in spite of 

 the abominable smell and taste, it is made still more disgusting, 

 and, to return to the camels, when the plants thus protect them- 

 selves, they, in their turn, are bound to develop a still greater 

 contempt for thorns and acrid or bitter juices. 



In the history of evolution, as told in the geological record, 

 nothing is more remarkable than the way in which a number of 

 groups move upward, keeping pace with one another. No one 

 largely outstrips the rest. The Silurian rocks tell of fishes but 

 of no vertebrates of higher rank. In the carboniferous period 

 appear a number of Labyrinthodonts, amphibians, some of them 

 with marked affinities to fishes, many of them giants compared 

 with our frogs and newts, running to 7 or 8 feet in length. In 

 Permian times reptiles begin. We have now done with the 

 primary rocks and enter the secondary period where reptiles 

 become dominant. There are many strange forms, great 

 Ichthyosaurians and Plesiosaurians, both frequenters of the sea 



