104 DARWINISM chap. 



throughout all geological time and in every part of the AV'orld. 

 Land and water have been continually shifting their positions ; 

 some reaiions are undergoing subsidence Avith diminution of 

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

 been converted into marshes, while mai'shes 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 

 Avhich connected continents and divided oceans. Again, along 

 AAdth 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 ; Avhile 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 jihysical changes as these have taken place, it is 

 eAddent that many species must either become modified or 

 cease to exist. AVhen the vegetation has changed in character 

 the herbivorous animals must become able to live on neAv and 

 perhaps less nutritious food ; Avhile the change from a damj) 

 to a dry climate may necessitate migration at certain periods 

 to escape destruction by drought. This AAdll expose the species 

 to ncAv dangers, and require special modifications of structure 

 to meet them. Greater SA\aftness, increased cunning, nocturnal 

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

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

 adopted by different species to bring themselves into harmony 

 Avith the neAV conditions ; and by the continued sui'viA^al of 

 those individuals, only, Avhich A^aried sufficiently in the right 

 direction, the necessary modifications of structure or of func- 

 tion Avould 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 AAald plant 

 such distinct forms as the cauliflower and the brussels sprouts. 



We will noAV consider the special characteristics of the 

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

 they agree Avith Avhat Ave observe in nature. 



