154 



THE AMERICAN NATURALIST 



[Vol. LVI 



stable, and on the other hand a changing- environment is 

 a stimulus to progressive changes in organisms. A few 

 animals have lived for ages in a stable environment. 

 Thompson cites the brachiopod, Ligula, as a " supreme 

 instance of static racial inertia." However, most animals 

 must live in environments that change. How do these 

 respond ? 



It is a matter of common knowledge that animal sys- 

 tems* of activities can become adapted to changes in the 

 environment, even when such changes constitute new 

 racial experiences. By taking increasing doses of certain 

 poisons at regular intervals animals develop enough im- 

 munity to be able to take daily a dose which in the begin- 

 ning would have been fatal. If a pigeon is fed nothing 

 but meat the lining of its stomach changes its character 

 and the bird's metabolic activities become adapted to an 

 unusual diet. Many other instances of acclimatization 

 to new conditions might be cited. 



Every physiographer knows that earth environments 

 change by succession. Land forms erode and water forms 

 fill up with sediment. Physiographic succession brings 

 about a succession of environments, or habitats. These 

 are successively occupied by different groups of plants 

 and animals and there is thus an ecological succession, 

 which is a succession of species or groups of species. 

 Shelford has worked out excellent examples of ecological 

 succession in the streams and ponds along the shore of 

 Lake Michigan. Pioneer species of animals invade hab- 

 itats soon after they are formed, and as the habitats 

 change the pioneer species are succeeded by others that 

 are adai)tod to later stages in physiographic succession. 

 Ecologic succession is a succession of species; animals 

 do not change as the environment changes, but die or 

 migrate to more favorable localities. Animals do not 

 appear to have special means for adapting themselves to 

 such changes. 



There are other types of succession, ]iow('\cr. to wliidi 

 animals show striking adaptation. The ty\>v> are all 

 rhythmic (seasonal, monthly and daily) and depend pri- 



