3i8 GENERAL BIOLOGY 



III. NATURE AND NURTURE. 



The germ cells constitute the bond between the genera- 

 tions. To the egg and the sperm we must look for sources 

 of hereditary characters. 



The human species inherits as do the other organisms. 

 Characters of various sorts "run in families;" form charac- 

 ters, such as shape of nose, of chin, of fingers; physiological 

 characters, such as left-handedness, baldness (in males), 

 slenderness or corpulence, etc. ; psychological characters, 

 such as emotional or judicial type of mind, phlegmatic or 

 effervescent temperament, etc. But most of these are 

 examples of complexes of characters, that must be analyzed 

 to their component units before their manner of inheritance 

 can be studied. To speak of infectious diseases as being 

 hereditary is wholly inaccurate, for disease germs are not 

 part of the body, but foreign organisms ; they can be passed on 

 from one generation to another only by infection, and not 

 by inheritance. There may, however, exist innate physio- 

 logical weakness that favors the infection in successive gener- 

 ations, and infection may occur before birth as well as after. 



However much the young may receive of fostering 

 parental aid in yolk, in shelter within or without the body, 

 in nourishment by means of embryonic membranes, etc., it 

 has already received when egg and sperm have united, its 

 full hereditary endowment; all else is nurture. 



Inheritance of acquired characters. In the lifetime of the 

 individual, the body may acquire various characters. The 

 skin may get a coat of tan in a few days exposure to the sun. 

 The hands become calloused with toil. The muscles 

 strengthen with use. Dexterity results from practice, and 

 by long effort we may acquire an education. But are any 

 of these things which the individual may acquire during his 

 lifetime heritable, or does the offspring start at the common 

 level of its kind, nothing advantaged by whatever his 



