HOW CHARACTERS ARE TRANSMITTED 127 



ovule with its pollen nucleus would develop but a single kernel, 

 and the operation described must be repeated for every one of 

 the thousand or more kernels of the ear, each of which for 

 present purposes is a distinct individual. 



The same is true for each grain of wheat, though in this case 

 the ovule and the pollen are produced in the same flower and 

 close together under the scale or chaff. So the process could 

 be traced for every seed of all species, for each is a new indi- 

 vidual. Among animals, also, but two nuclei are involved for 

 each new individual whether as small as the cricket or as large 

 as the elephant. 



Little enough is known of the essential constitution of these 

 remarkable bits of living matter called nuclei, but that little is 

 too much to discuss exhaustively here. 1 It is enough for present 

 purposes to call attention to the wonderful fact that these two 

 bits of matter, too small to be seen and studied with the naked 

 eye, carry with them all the characters of the race ; moreover, 

 as they constitute the only material transmitted from parent to 

 offspring, they are the only vehicles of transmission. Other 

 nuclei from other parts of the body can repeatedly divide, absorb- 

 ing food as they do so, constituting growth, but these nuclei 

 from the reproductive cells, excepting in certain lower species, 

 do not grow till after union with others from the opposite sex. 



Chromosomes. The nucleus of the animal or plant cell is 

 something more than a formless bit of matter endowed with 

 life. If the nuclei of several species be stained and examined 

 under a high-power microscope, each will be found to contain 

 a definite number of rods, rings, or other bodies, always the 

 same in all the cells of all the individuals of the same species, 

 but differing in different species. These are called chromosomes. 



Another peculiarity about the chromosomes is that for all 

 species that propagate bisexually the number is even ; thus in 

 mouse, trout, and lily it is 24 ; in ox, guinea pig, onion, and 



1 It may be conveniently pursued further in " Principles of Breeding," chaps, 

 vii and viii. 



