126 Hereditary Factors or "Genes" 



how much mutation there may have been in many 

 millions of years ! 



In some cases it is certain that the germinal repre- 

 sentatives ("factors" or "genes") corresponding to 

 brusque new departures or mutations, and to all or 

 many of the discrete non-blending "unit characters" 

 of the organism, lie in linear order — like beads on a 

 string — in the nuclear rods or chromosomes of the 

 germ-cells, and in a few cases something is known of 

 their topography. 



In the fruit-fly Drosophila there are about 7500 

 of these germinal "factors" or "genes," and a new 

 light has been thrown on variability by the realiza- 

 tion that these are like hereditary cards which are 

 re-shuffled at the beginning of each new life. It has 

 been shown that one hereditary factor may affect 

 several adult characters (thus corroborating Dar- 

 win's idea of the correlation of variations), and that 

 one adult character or quality may be the outcome 

 of several hereditary factors. 



From the study of some fossil series, such as certain 

 lineages of Ammonites, Lamp-shells, crocodiles, and 

 elephants, and also from some experiments, there is 

 evidence that variations are sometimes definite or 

 orthogenetic — i.e., occurring persistently in a par- 

 ticular direction. In other words, a variation may be 

 congruent with what has already been enregistered 

 in the organization of the creature. If an architect 

 is asked to enlarge and improve a building, he cannot 

 add at random, but only what is consistent with the 

 given style. So it is unlikely that "anyhow" varia- 



