CHAPTER VII 

 THE MECHANISM OF ADAPTATION 



1. The Adaptation as a Reversible Change 



The close adaptations of animals and plants to their individual 

 stations in life, fitting them to secure food and protect themselves, 

 are the most striking features we find in them. Since environment is 

 usually not constant, it is plain that if living things are to continue 

 to live, they must have the capacity for readjustment directly or in- 

 directly to changed environment. 



It is a matter of common observation that plants of the same 

 species, even if grown from seeds of the same fruit, may be quite unlike 

 when grown in very different habitats. Thus, when plants of moist 

 valleys are grown from seed or by transplantation in alpine regions, 

 their internodes are generally shorter, their leaves are smaller and 

 thicker and their flowers are larger than when they grow in their 

 natural habitat. If, however, the seeds of these plants are planted in 

 the natural habitat again, it will be found that the modifications 

 -which the species displayed when grown in an alpine habitat have 

 not been inherited. The reversibility of the change is thus definite. 



If we continue to use some of our muscles in excess, by and by 

 the muscle will become hypertrophic so that we may be able to use 

 it with facility. Such an adaptation is especially striking with bones 

 and blood vessels as well as with muscles, but the change is reversible 

 and the removal of the stimuli will soon be followed by the recovery 

 of the normal state. 



Some animals like chameleons of the tropics can change color 

 promptly according to the color of the background. Some of the most 

 remarkable instances of variable concealment coloration are met with 

 in the flounders and various other bottom-dwelling species of fish 

 which change their colour pattern in conformity with the colour shade 

 of the background. According to Kammerer (69) the salamander, 

 Saramandara maculosa, could change their colour pattern in a similar 

 way, but the change proceeded very gradually during the prolonged 

 breeding periods with the background of certain colours. The obser- 

 vation was continued for several years, but the change was not 

 definitely proved to be heritable. As is generally recognized, the colour 



