REPRODUCTION. 931 



cation to the human race it would seem that the factors of social evolution 

 have brought it about that the aged are protected in the struggle for-existence 

 for long after their reproductive usefulness has ceased, and thus the working 

 of a pitiless biological law has become modified. 



F. HEREDITY. 



Biologists are accustomed to recognize two factors as responsible for the 

 character and actions of the living organism. These are heredity and the 

 environment. Heredity includes whatever is transmitted, either as actual or 

 as potential characteristics, by parents to offspring. The .environment com- 

 prises both material and immaterial components, such as food, water, air, or 

 other substances that surround the organism, and the forces of nature, such as 

 light, heat, electricity, and gravity, that act as conditions of existence or as 

 stimuli to action. The same principles apply to the character and actions of 

 every cell of a many-celled organism, but here we must include in the envi- 

 ronmental factor the mysterious influences that are exerted upon the cell by 

 the other cells of the body. Of these two factors heredity acts from within, 

 the environment from without the living substance. Among unicellular or- 

 ganisms the individual begins its career when the bit of protoplasm that con- 

 stitutes its body is separated from the parent bit of protoplasm. Among 

 higher forms, including man, the term individual may be applied to the fer- 

 tilized ovum ; the union of the ovum and the spermatozoon inaugurates the 

 new being. From the inception to the death of the individual, life consists 

 partly of manifestations of the powers conferred by the germ-cells and partly 

 of reactions to environmental influences. In considering the details of vital 

 action we are apt to overlook these fundamental facts and to evolve narrow 

 and erroneous views as to the causes of vital phenomena. Biologists are 

 seeking with increasing vigor to determine the relative importance of the parts 

 played by these two principles in development and in daily life. It is need- 

 less to say that the problem is a difficult one and is still far from solution. 

 In previous chapters of this book attention has been directed more especially 

 to the external than to the hereditary factor. A work upon physiology would 

 be incomplete, however, if it did not include an examination of the latter, 

 especially since at the present time heredity is one of the leading subjects of 

 biological research and discussion. It is proposed, therefore, in this section 

 to present a brief outline of the facts, the principles, and the attempted ex- 

 planations of the modes of working of heredity. It should be premised that, 

 because of the present incomplete state of our knowledge of the facts, the 

 highly speculative and involved character of most of the theories, and the con- 

 stant, active shifting of ideas and points of view, such an outline must neces- 

 sarily be incomplete and in many respects unsatisfactory. 



Facts of Inheritance. It is not proposed in this paragraph to enter into 

 a discussion of the question as to whether a particular vital phenomenon is a 

 fact of inheritance or a reaction to external influences. For our present pur- 

 poses it is sufficient to record the common facts of resemblance to ancestors, 



