494 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 tacts and to evolve narrow 

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

 -ccking 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- 

 le-- 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. 



Pacts 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, 

 and to assume that such resemblance, when present, has been inherited. 

 Resemblances are strongest between child and parents, and appear in a dimin- 

 ishing ratio backward along the ancestral line. Galton l has computed that, 

 of the total heritage of the child, each of the two parents contributes one- 

 fourth, each of the four grandparents one-sixteenth, and the remaining one- 

 fourth is handed down by more remote ancestors. The correctness of this 

 estimate has been disputed by Weismann. The fact must not be overlooked 

 that, in addition to and back of all the particular individual features that are 

 inherited, a host of racial characteristics are transmitted — the progeny of a 

 given species belongs to that species; the human being is the father of the 

 human child, the child of Caucasian parents is a Caucasian, of negro parents 

 a negro. 



Congenital resemblances may be anatomical, physiological, or psychological, 

 and in each of these classes they may be normal or pathological. Anatomical 

 resemblances are the most commonly recognized of all : facial features, stature, 

 color of eves and of hair, supernumerary digits, excessive hairiness of body, 

 cleft palate, monstrosities, and various defects of the eye, such as those that 

 give rise to hypermetropia, myopia, cataract, color-blindness, and strabismus, 

 are all known examples. Physiological peculiarities that may be transmitted 

 include the tendency to characteristic gestures, locomotion and other muscular 

 movements, longevity or short life, tendency to thinness or obesity, handwriting, 

 1 Francis Galton : Natural Inheritance, 1889, p. 134. 



