ic6 THE POPULAR SCIENCE MONTHLY 



merely as figures of speech, but when interpreted literally, as they 

 frequently are, they are altogether misleading; they are the result of 

 reasoning about names rather than facts, of getting far from phenomena 

 and philosophizing about them. The comparison of heredity to the 

 transmission of property from parents to children has produced con- 

 fusion in the scientific as well as in the popular mind. It is only neces- 

 sary to recall the most elementary facts about development to recognize 

 that in a literal sense parental characteristics are never transmitted 

 to children. 



2. The Transmission Hypothesis 



And yet the idea that the characteristics of adult persons are trans- 

 mitted from one generation to the next is a very ancient one and was 

 universally held until the most recent times. Before the details of 

 development were known it was natural to suppose, as Hippocrates did, 

 that white-flowered plants gave rise to white-flowered seeds and that 

 blue-eyed parents produced blue-eyed germs, without attempting to 

 define what was meant by white-flowered seed or blue-eyed germs. And 

 even after the facts of development were fairly well known it was 

 generally held that the germ cells were produced by the adult animal or 

 plant and that the characteristics of the adult were in some way carried 

 over to the germ cells; but the manner in which this supposed trans- 

 mission took place remained undefined until Darwin attempted to 

 explain it by his "provisional hypothesis of pangenesis." Darwin 

 assumed that minute particles or "gernmules" were given off by every 

 cell of the body, at every stage of development, and that these gernmules 

 then collected in the germ cells which thus became storehouses of little 

 germs from all parts of the body. Afterwards, in the development of 

 these germ cells, the gernmules, or little germs, developed into cells and 

 organs similar to those from which they came. 



3. Germinal Continuity and Somatic Discontinuity 

 Many ingenious hypotheses have been devised to explain things 

 which are not true, and this is one of them. The doctrine that adult 

 organisms manufacture germ cells and transmit their characters to 

 them is known to be erroneous. Neither germ cells nor an} r other kind 

 of cells are formed by the body as a whole, but every cell in the body 

 comes from a preceding cell by a process of division, and germ cells are 

 formed, not by contributions from all parts of the body, but by division 

 of preceding cells which are derived ultimately from the fertilized egg 

 (Fig. 23). The hen does not produce the egg, but the egg produces 

 the hen and also other eggs. Individual traits are not transmitted from 

 the hen to the egg, but they develop out of germinal factors which are 

 carried along from cell to cell, and thus from generation to generation. 

 There is a continuity of germinal substance, and usually of germi- 

 nal cells, from one generation to the next. In some animals the germ 

 cells are set apart at a very early stage of development, sometimes in 



