14 THE PHYSICAL BASIS OF LIFE 



ity. The pioneer investigations of Hertwig, Strasburger, 

 Van Beneden and Boveri long since produced strong evi- 

 dence that the nucleus of the cell has an especial signifi- 

 cance in the operations of heredity. Sutton had shown 

 that the more obvious of the Mendelian phenomena, at 

 first sight seemingly so inscrutal^le, find a simple and in- 

 telligible explanation in the combinations, segregations 

 and recombinations of the chromosomes. Be Vries, Cor- 

 rens and Strasburger had urged the importance from the 

 genetic point of view of Boux's conception of the chromo- 

 somes as representing linear series of smaller bodies that 

 are somehow concerned with the determination of partic- 

 ular hereditary characters ; but these writers made no ap- 

 proach to an adequate interpretation of the phenomena, 

 even as then known."' The subject first began to take on 

 more definite form with the initial experimental investi- 

 gations of Morgan and of Sturtevant, which brought very 

 strong support to the conception of the chromosomes as 

 linear aggregates, showing in particular that this concep- 

 tion affords a complete explanation of the genetic phe- 

 nomena known as linkage and recombination l)y "cross- 

 ing-over." For all this the way had in some degree been 

 prepared by earlier investigations; but the further con- 

 clusions which soon followed seemed at first sight com- 

 pletely incredible. Step by step the experimental analysis 

 ])uilt up the demonstration that the infinitesimal entities 

 serially aligned in the nuclear threads are primary and 

 indivisible units or factors of heredity ("genes"), each of 

 its own specific kind and self-perpetuating l)y growth and 

 division ; that they are of definite iuunl)er ; that they are 

 separated by fairly definite and constant intervals; and 

 that in their serial alignment tlieij follow a definite and 

 invariable order! When we try to reckon with this se- 

 ries of conclusions, we find ourselves fairly gasping for 



