36 BIOLOGICAL SOCIETY OF WASHINGTON. 



ovum contains the germ-plasms of the two parents, and out of 

 it the embryo is formed. The embryo develops independently 

 of the mother by a circulation of its own, and no-external in 

 fluences can by any conceivable method affect or change the 

 characters of the offspring. 



But it is well known that variation takes place, that the off 

 spring does not always resemble either parent, and that changes 

 go on so great as to result in the creation of new species, new 

 genera, and entirely new types of life. All this Weismann ad 

 mits. How does he explain it ? Primarily by natural selec 

 tion, but he does not stop with that. It has always been ad 

 mitted that natural selection did not explain the cause of varia 

 tion. Weismann attempts to do this, and his reasoning is 

 exceedingly ingenious. 



The original reproductive cells are assumed by him to con 

 sist of an indefinite number of units which he calls germ-plasms, 

 and their presence is explained on the assumption of their pre 

 servation from ancestral organisms. Asexual reproduction is 

 of course incapable of producing variation, and he maintains 

 that sexual reproduction has been developed and exists solely 

 for the purpose of insuring variation . 



Relative to the constitution of the germ-plasm he says : 

 ' * Every detail in the whole organism must be represented in the 

 germ-plasm by its own special and peculiar arrangement of the 

 groups of molecules (micellae of Nageli) and the germ-plasm 

 not only contains the whole of the quantitative and qualitative 

 characters of the species, but also all individual variations as 

 far as these are hereditary : for example the small depression 

 in the center of the chin noticed in some families. The physi 

 cal causes of all apparently unimportant hereditary habits or 

 structures, of hereditary talents, and other mental peculiarities, 

 must all be contained in the minute quantity of germ-plasm 



