96 Introductory Biology 



tiation, the cells and tissues are subjected to still further variations, in 

 this case possibly by even the same environmental influences, because 

 now the tissues are even different. It has been experimentally observed 

 that the embryologic development of a cell or tissue is influenced by its 

 inherent abilities (genes, etc.) and by the particular environment (chemi- 

 cal and physical) to which it is subjected. Both of the phenomena are 

 considered in greater detail in other chapters. 



V. REPRODUCTION 



Reproduction, or the ability to reproduce themselves, is a character- 

 istic of living organisms. Most offspring are remarkably like their par- 

 ents, which suggests there are similar forces which operate generation 

 after generation in order to ensure this phenomenon of continuity. 

 Naturally, certain off^spring differ somewhat from their parents, and the 

 explanations are to be found in differences in the heredity mechanism 

 or differences in environmental factors, or a combination of both. The 

 various methods of reproduction of plants and animals, their embryo- 

 logic developments, the effects of environmental factors, and the opera- 

 tions of heredity (genetics) are considered in other chapters. Reproduc- 

 tion is essential if individuals are to propagate their kind and if the race 

 is to continue existence. The specific methods of reproduction in plants 

 and animals are numerous and varied as a study of living organisms 

 will reveal. 



VI. ADAPTATION AND IRRITABILITY 



Adaptability is the ability of living organisms to undergo changes ap- 

 propriate to their life needs and to fit efficiently into their environments 

 so that their life processes may proceed as normally and effectively as 

 possible. Adaptiveness of living organisms depends upon their irrita- 

 bility (their capacity to respond to stimuli). Nonliving things may be 

 more or less affected by external influences, although their reactions are 

 always more or less the same, while living things do not always react 

 to the same stimuli in the same way. These differences in reactions are 

 dependent on the adaptability of the living protoplasm. Naturally, the 

 specific or definite response to a particular type of stimulus cannot 

 always be predicted in a living organism as it can in nonliving materials. 

 Adaptations of living organisms may be of three kinds: (1) changes in 

 structure (structural); (2) changes in functions (functional); (3) 

 changes in both structure and function (structural-functional). 



