104 DARWINISM 



throughout all geological time and in every part of the world. 

 Land and water have been continually shifting their positions ; 

 some regions are undergoing subsidence with diminution of 

 area, others elevation with extension of area ; dry land has 

 been converted into marshes, while marshes have been drained 

 or have even been elevated into plateaux. Climate too has 

 changed again and again, either through the elevation of 

 mountains in high latitudes leading to the accumulation of 

 snow and ice, or by a change in the direction of winds and 

 ocean currents produced by the subsidence or elevation of lands 

 which connected continents and divided oceans. Again, along 

 with all these changes have come not less important changes 

 in the distribution of species. Vegetation has been greatly 

 modified by changes of climate and of altitude ; while every 

 union of lands before separated has led to extensive migrations 

 of animals into new countries, disturbing the balance that 

 before existed among its forms of life, leading to the extermina- 

 tion of some species and the increase of others. 



When such physical changes as these have taken place, it is 

 evident that many species must either become modified or 

 cease to exist. When the vegetation has changed in character 

 the herbivorous animals must become able to live on new and 

 perhaps less nutritious food ; while the change from a damp 

 to a dry climate may necessitate migration at certain periods 

 to escape destruction by drought. This will expose the species 

 to new dangers, and require special modifications of structure 

 to meet them. Greater swiftness, increased cunning, nocturnal 

 habits, change of colour, or the power of climbing trees and 

 living for a time on their foliage or fruit, may be the means 

 adopted by different species to bring themselves into harmony 

 with the new conditions ; and by the continued survival of 

 those individuals, only, which varied sufficiently in the right 

 direction, the necessary modifications of structure or of func- 

 tion would be brought about, just as surely as man has been 

 able to breed the greyhound to hunt by sight and the fox- 

 hound by scent, or has produced from the same wild plant 

 such distinct forms as the cauliflower and the brussels sprouts. 



We will now consider the special characteristics of the 

 changes in species that are likely to be effected, and how far 

 they agree with what we observe in nature. 



