138 DARWINISM AND HUMAN LIFE 



apart, it is by the minute germ-cells that the 

 secret of life and the character of each kind of 

 living creature are sustained from generation to 

 generation. Within every cell in the body of 

 an organism there is usually a nucleus, and within 

 the nucleus a number — a definite number — of 

 readily stainable rods, or loops, or grains called 

 chromosomes. Each kind of living creature has 

 a definite number ; thus, there are thirty-two in 

 man and in the cockroach, twenty-four in mouse 

 and in lily, twelve in the grasshopper, and two in 

 a species of threadworm. There is no doubt that 

 these stainable bodies, or chromosomes (including 

 less visible bodies associated with them), are very 

 important. There are many facts pointing to the 

 conclusion that they are bearers (not perhaps the 

 exclusive bearers) of specifically different materials 

 which, in appropriate conditions, will develop into 

 particular heritable qualities. One of the leaders 

 of experimental zoology, Dr. Przribram, sums up 

 a number of remarkable investigations when he 

 says : " Substances or parts can be actually 

 demonstrated in the ovum, the removal of which 

 conditions the absence of definite organs or parts in 

 the embryo." Now, while the immature germ-cells 

 have the same number of chromosomes as the 

 cells of the body have, the mature germ-cells have 

 half the normal number. If 8 be the normal 

 number, the ripe ovum has 4, and the fully formed 

 spermatozoon has 4, so that when the ovum is 

 fertilised the normal number is restored. In a 

 remarkable way, by a kind of cell-division which 

 occurs only in the maturing germ-cells, the number 

 of chromosomes is always reduced by a half— 

 except, indeed, in certain cases of parthenogenesis. 



