ORIGIN OF TH LIVING MACHINE: ADAPTATION 355 



will be slightly better adapted than the last. By an accumula- 

 tion of the improvements which thus appear accidentally, there 

 will be developed, as the generations pass, a closer and closer 

 adaptation to conditions. The final result is a better adaptation 

 to conditions, and a gradual change of type and production of 

 new species. 



Acquired and Congenital Characters Affecting Natural 

 Selection. In the form stated above, and as at first conceived 

 by Darwin, the characters which are chosen by natural selection, 

 and upon which the advance of the race is based, might be either 

 acquired characters, such as those upon which Lamarck based 

 his theory, or they might be congenital characters, which are in 

 the germ plasm and essentially due to variation in the heredi- 

 tary substance. Darwin did not sharply separate these two 

 types of variation, although he recognized them both. Dar- 

 win thought that the advancement of type was produced prima- 

 rily by the natural selection of such characters as were born with 

 the individual, i. e., congenital characters. He also believed 

 that, to a certain extent, acquired characters, which were pro- 

 duced in the animal either by the direct effect of the environ- 

 ment or by use or disuse, could be transmitted and might thus 

 affect posterity and have an influence in changing the type. 

 Darwin did not believe, as did Lamarck, that these acquired 

 characters were the primary factors in producing divergence of 

 type, but thought they might be secondary ones, the primary 

 factor being the selection of most favorable congenital varia- 

 tions. 



Weismann. The discussion of Darwin's theories continued 

 vigorously for a quarter of a century, until his views of descent 

 were quite generally accepted, although with various opinions 

 as to the efficiency of his law of natural selection. In 1884 

 appeared the essay of Weismann "On Heredity," which put a 

 totally new aspect on the whole problem. His theory of hered- 

 ity, already described, was so simple, and so readily obtained 

 confirmation by direct observation, that it soon acquired almost 



