PARTICULATE THEORIES OF HEREDITY 29 



gemmiiles, are being continually tbroAvn off from every 

 part of the body. Those that reach the germ-cells become 

 incorporated there with the hereditary units of the same 

 general kind already present. 



The theory was proposed primarily to explain how 

 acquired characters are transmitted. If specific changes 

 in the body of the parent are transmitted to the offspring, 

 some such theory is required. If the changes in the body 

 are not transmitted, there is no need of such a theory. 



Weismann in 1883 challenged the entire transport 

 theory, and convinced many, but not all, biologists that 

 the evidence for the transmission of acquired characters 

 was inadequate. This led him to develop his theory of 

 the isolation of the germ-plasm. The egg produces not 

 only a new individual, but other eggs like itself, carried 

 by the new individual. The egg produces the individual, 

 but the individual has no subsequent influence on the 

 germ-plasm of the eggs contained in it, except to protect 

 and to nourish them. 



From this beginning Weismann developed a theory of 

 particulate inheritance of representative elements. He 

 appealed to evidence derived from variation, and he ex- 

 tended his theory to include a purely formal explanation 

 of embryonic development. 



We are concerned, in the first place, mth Weismann 's 

 views as to the nature of the hereditary elements or ids 

 as he calls them. The ids he identified in his later writings 

 as small chromosomes when many small chromosomes 

 are present, but when only a few chromosomes are pres- 

 ent he supposed that each is made up of several or many 

 ids. Each id contains all the elements that are essential 

 to the development of a single individual. Each is a mi- 

 crocosm. The ids differ from each other in that they are 

 the representatives of ancestral individuals or germ- 



