144 MEANING OF THE SYSTEM. 



of animal forms, and of the results of artificial selection, had already, 

 in the beginning of this century, induced illustrious and highly 

 esteemed naturalists to dispute the dominant views on the immuta- 

 bility of species. In the year 1809, Lamark, in his " Philosopkie 

 Zoologique" broached the theory of the descent of species from one 

 another. He referred the gradual alterations in some degree to 

 changing conditions of life, but mainly to use and disuse of organs. 



Geoffrey St. Hilaire, too, the advocate of the idea of unity of 

 organization of all animals and the opponent of Cuvier, expressed 

 his conviction that species had not existed unaltered from the be- 

 ginning. While agreeing essentially with Lanark's theory of the 

 origin and transmutation of species, he ascribed a less influence to 

 the inherent activity of the organism, and believed that he could 

 explain the alterations through the direct operation of changes in 

 the environment (monde ambiant). 



The change in the fundamental views of Geology which took 

 place at a later period must be ascribed to the opinions of these 

 investigators. Lyell endeavoured (Principles of Geology) to explain 

 geological alterations by means of the forces in operation at the 

 present day, working gradually and without interruption through 

 extended periods of time, and rejected the Cuvierian theory of 

 mighty revolutions and catastrophes which destroyed all life. When 

 the geologists with Lyell had given up the hypothesis of periodic 

 disturbance of the course of natural events, they were obliged to 

 assume the continuity of organic life during the successive periods 

 of the formation of the earth, and to endeavour to account for the 

 immense alterations of the organic world by slight influences operat- 

 ing gradually and without interruption throughout long periods of 

 time. The variability of species, the origin of new species from 

 previous ancestral forms in the course of ages, has become, accord- 

 ingly, since the time of Lyell, a necessary postulate of geology in 

 order to explain naturally the differences of animals and plants 

 in successive periods without the supposition of repeated acts of 

 creation. 



THE TRANSMUTATION THEORY, OR THEORY OF DESCENT, BASED ON 

 THE PRINCIPLE OF NATURAL SELECTION (DARWINISM). 



Nevertheless a more securely grounded theory based upon a firmer 

 standpoint was needed in order to give more force to the Trans- 

 mutation hypothesis which had remained disregarded ; and this 



