Fitness in the Living World 3 1 9 



equality and the halves separated into the two daughter 

 cells formed at each division. This "purpose" is accom- 

 plished by the complex mechanism of mitosis, which is al- 

 most universal in occurrence and has existed at least as long 

 as many-celled animals and plants have, but which was not 

 discovered by man until the middle of the nineteenth cen- 

 tury, and its significance has been appreciated only during 

 the past forty years. 



Since chromosomes are persistent structures their number 

 would double, and the inheritance material would double, 

 every time a spermatozoon unites with an egg, were not 

 some provision made to prevent this. Such provision is 

 made in a unique form of nuclear division, which takes 

 place only once in the whole life of an individual in man 

 once in billions of billions of divisions. This unique division 

 is brought about by the union of corresponding or homolo- 

 gous chromosomes of the father and mother at the time of 

 the formation of the germ-cells a process known as synap- 

 sis and the subsequent separation of these whole chromo- 

 somes in mitosis, so that each germ-cell, whether egg or 

 sperm, contains only half the normal number ; then when egg 

 and sperm unite in fertilization, the normal number is re- 

 stored. Upon these processes of synapsis and reduction 

 of chromosomes and subsequent union of egg and sperm 

 depend all the phenomena of Mendelian inheritance. 



The fact that in most species males and females occur in 

 equal numbers has always been regarded as a remarkable 

 adaptation indeed, the fact that males or females, with 

 their coadapted structures, functions, and instincts, should 

 occur at all is a notable adaptation. It is now known that sex 

 is determined by a certain combination of chromosomes; in 

 the female there are usually two sex chromosomes (xx), in 

 the male there is only one (#), or a combination (xy) In 



