THEORIES OF EVOLUTION 155 



is to be understood, therefore, that as regards Dar- 

 winism, his is a supplementary or supporting theory 

 and not a replacing theory. 



It is complex and involves assumptions as to the 

 behavior of a number of hypothetical vital units, 

 designated by Weismann : idants (chromosomes) ; ids 

 (chromomeres) , determinants, and biophors (the 

 elementary vital units). Two of these, the ids and 

 the idants are visible under the microscope, but the 

 determinants and the biophors are too minute to be 

 rendered visible. 



His theory of evolution is in reality the outcome of 

 his theory of heredity, designated the "Continuity 

 of germ-plasm," and to comprehend his reasoning it is 

 necessary to understand what is meant by the germ- 

 plasm and by its continuity. 



As is well known, animals and plants arise from 

 germinal elements of microscopic size; these are, in 

 plants, the spores, the ovules and their fertilizing 

 agents; and, in animals, the eggs and the sperms. 

 Now, since all animals, even the highest, begin their 

 existence as a fertilized egg, that structure, minute as 

 it is, must contain all hereditary qualities, because 

 this is the only material substance that passes from 

 one generation to another. This formative sub- 



