12 ASSAYS SCIENTIFIC AND PHILOSOPHICAL. 



Apart from the other difficulties of this theory, it 

 has been practically proved by Galton's experi- 

 ments that the "gemmules" do not exist, and, 

 moreover, the tendency of recent science has been 

 almost entirely in the direction of emphasizing the 

 importance of the Darwinian principle of natural 

 selection, and excluding what remains of Lamarck- 

 ianism in the doctrine of evolution. 



Professor Weismann's theory of heredity is the 

 latest development of this tendency, (i.) It explains 

 heredity by the continuity of the germ-plasm ; 

 (ii.) it denies the possibility of the inheritance of 

 acquired characters ; and (iii.) it recognizes in 

 natural selection the sole factor in the evolution of 

 species, at least among the metaphyta and metazoa. 



The continuity of the germ-plasm. We have seen 

 that the problem of heredity does not really present 

 itself in those plants and animals which increase 

 by fission or division. For here the parts are at 

 the time of the division alike because identical, and 

 retain their likeness except when acted upon by 

 the environment. But when sexual reproduction 

 appears, the offspring is no longer identical with 

 either parent, but combines in varying proportions 

 the characteristics of both, though we can trace it 

 back to a microscopic cell. This microscopic cell 

 must contain all that grows from it not like a 

 Chinese puzzle box-within-box (emboitement\ nor 

 as containing preformed gemmules (j>angtnesis\ 



