THEORIES OF HEREDITY 5 



cells, in the different lines of descent, do not multiply at the 

 same rate as in man. 



9. All evolution consists, in essence, of a gradual change in v 

 the character of the germ-plasm contained in successive germ- 

 cells a change of such a nature that it is reflected in the 

 successive individuals that spring from the cells. Thus, if, by 

 careful breeding, we succeed in altering any plant or animal 

 species, it is only because we have succeeded in altering to a 

 corresponding extent the nature of the germ-plasm of that 

 species. 



10. All the traits or characters (the terms are synonymous) 

 of every living being are separable in theory, if not always in 

 practice, into two distinct categories. Either they are inborn 

 or they are acquired. The first essential to a study of here- 

 dity is a clear comprehension of the distinction between inborn 

 and acquired characters. Inborn traits or characters are those 

 which take origin in the germ-plasm, which arise because the 

 germ-plasm is so constituted that it tends, under fit conditions 

 of shelter and nutrition, to impel or cause the germ-cell to 

 proliferate into an individual having those characters. Thus 

 a man's head is inborn. It arises because the germ-plasm in 

 the fertilized ovum whence he sprang was so constituted that 

 it impelled that germ-cell to proliferate into a being having 

 a man's head. Acquired characters, on the other hand, do not 

 take origin in the germ-plasm ; they are modifications -of inborn 

 characters caused by the play of forces from the environment on 

 those characters after (as a rule) they have developed from the 

 germ-cell. Thus a man's hand is inborn. But if it be modi- 

 fied by use, disuse, accident, or the play of other forces from 

 the environment the modification is an acquired character. 1 

 Inborn characters take origin in the germ ; that is, they are 

 blastogenetic in origin. They express the hereditary tend- 

 encies of the individual, and, with variations, those of the 

 race. Acquired characters take origin (as a rule) in the cell- 



1 " All the effects of exercise are acquirements for example, the enlarge- 

 ment which exercise causes in muscles. The effects of lack of exercise are 

 also acquirements for example, the wasting of a disused muscle. The 

 effects of injury are acquirements for example, the changes in a diseased 

 lung or an injured arm. Every modification of the mind is also an 

 acquirement for example, everything stored within the memory. If a 

 man be blinded by accident or disease his blindness is acquired. But 

 if he come into the world blind, if he be blind " by nature," his blind- 

 ness is inborn. If a son be naturally smaller than his father, his in- 

 feriority of size is inborn ; but if his growth be stunted by ill-health or 

 lack of nourishment or exercise, his inferiority is acquired." (Alcoholism. 

 A Study in Heredity, p. 9.) 



