RE PR OB UCTION. 493 



day, with a continuance of the proper nutritive conditions protoplasm does not 

 grow old and die; the single individual divides into two and life continues 

 unceasing, unless accident or other untoward event interferes. With the 

 progress of evolution, however, the cells of the individual body have become 

 differentiated into germ-cells and somatic cells, the former subserving the 

 reproduction of the species, the latter all tl ther bodily functions. Germ- 

 cells are passed on from parent to offspring; they never die, they arc immor- 

 tal. Somatic cells, on the other hand, grow old, and at last perish. Death 

 was, therefore, in the beginning, not a necessary adjunct to life ; it is not inhe- 

 rent in primitive protoplasm, but has been acquired along with the differen- 

 tiation of protoplasm into germ-plasm and somatoplasm, and the introduction 

 of a sexual method of reproduction. It has been acquired because it is to the 

 advantage of the species to possess it; in the simplest cases it should occur at 

 the close of the reproductive period, and in fact it frequently does occur then. 

 A superabundance of aged individuals, after they have ceased to be reproduc- 

 tive, would be detrimental to the race ; it is to the advantage of the species that 

 they be put out of the way. Death of the individual in order that the species 

 may survive has, therefore, become an established principle of nature. But 

 the higher animals are better able to protect themselves from destruction than 

 the lower, and, moreover, they are needed to rear the young ; hence in them 

 the duration of life is frequently prolonged beyond the reproductive period. 



Weismann's theory has been the cause of much discussion, and the pros 

 and cons have been set forth by eminent biological authorities. In its appli- 

 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 his career when the bit of protoplasm thai con- 

 stitutes his 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 



