256 NATURAL HISTORY. 



organs becoming thereby more developed, while others 

 diminish or altogether disappear. In this case, however, 

 changed circumstances operate by inducing changes in 

 the nutrition of the plant, 'in its processes of absorption 

 and transpiration, in the quantity of heat, light, air, and 

 moisture which it habitually receives, finally in the pre- 

 dominance which may be established in some of its vital 

 movements over others.' Lamarck brings forward many 

 instances of the changes produced in plants by changed 

 circumstances, and expresses the opinion that in plants 

 such changes are more rapidly brought about than in 

 animals; since the causes which affect animals operate 

 very slowly, and are consequently difficult of appreciation 

 and recognition by us. 



With regard to the nature of the circumstances which 

 are mainly operative in producing changes in animals, 

 Lamarck assigns the first place to the character of the 

 medium in which it lives whether it be terrestrial, aerial, 

 or aquatic in its habits. He also attaches great import- 

 ance to the differences of temperature, moisture, and the 

 like in different regions, and thinks that such differences 

 are a great cause of variation in animals and plants. 

 Every one, he says, admits these great climatic differences ; 

 but 'what is not sufficiently recognised, or is even 

 altogether denied, is that each place itself is liable in the 

 course of time to changes of its climatic conditions; 

 these changes being effected with such slowness, in 

 relation to our lifetime, that we attribute to the existing 

 conditions a perfect stability? To this he elsewhere 

 adds that 'we may be sure that this appearance of 

 stability in natural things will always be taken, by the 



