290 ELEMENTARY BIOLOGY 



other " freak " forms have been produced as a result of chang- 

 ing the external conditions of development. 



337. Internal factors. In considering the conditions under 

 which development takes place, we have given attention chiefly 

 to the evidence that exceptional conditions will interfere with 

 or modify what we usually consider the normal course of 

 development. We must be on our guard, however, in respect 

 to two points : 



1 . Whatever the possibilities of an egg, these possibilities can 

 become realities only under certain definite series of external 

 conditions. Thus, the egg of the frog or the fish must be in water 

 if it is to develop ; the egg of the hen must be kept above a 

 certain temperature if it is to develop at all ; and so on. 



2. But the external conditions are not to be thought of as 

 the causes of the development, for by themselves they are not 

 the causes. The protoplasm inside a hen's egg, with the water 

 and salts and food in the egg, with the air that circulates 

 through the shell, with the temperature all these factors 

 together cause the becoming of the chick. The fundamental 

 and distinctive factors are in the minute, invisible parts of the 

 protoplasm. The external conditions make the development 

 possible, but the nature of the plant or animal, or rather the 

 organic possibilities of the organism, are already present in 

 the protoplasm. 



External conditions are related to development in several 

 different ways : 



1. They may supply materials essential to the activities 

 of the protoplasm for example, growth. 



2. They may supply conditions for the chemical action of 

 the parts of the protoplasm, as moisture, heat, light. 



3. They may stimulate the activities of certain parts of the 

 developing organism or certain of the chemical processes in 

 the protoplasm. 



4. They may retard certain of the activities or processes, 

 or they may prevent certain activities. 



