114 GENERAL ZOOLOGY 



see p. 210), which bear only the pure character of one 

 parent or the other, but never both ; and these will be pro- 

 duced in equal numbers. This is known as " the law of the 

 purity of the germ-cells." From this law follows the occur- 

 rence in the next and succeeding hybrid generations of a 

 definite number of forms in definite numerical proportions. 

 Thus in the third generation of rabbits, produced by the 

 breeding together of the progeny of the gray and the albino 

 rabbits already referred to, there are, in nearly every case, 

 three gray young to one albino. Results similar to these 

 were obtained by Professor Castle with mice and guinea- 

 pigs also. 



The explanation of why a character may manifest itself in 

 the third generation, after being invisible in the second, is 

 to be found in the theory that characters of organisms are 

 transmitted from generation to generation only in the germ- 

 cells, and by extremely minute bodies, called chromosomes, 

 within these cells. The second generation is all gray, because 

 in the united germ-cells from male and female parents the 

 chromosomes which bear the gray character are more powerful 

 to express themselves in the appearance of the young which 

 is produced than are those chromosomes which bear the 

 albino character. However, the albino chromosomes are not 

 destroyed, but are carried along with the others and give 

 rise to other chromosomes of their own kind, until, in the 

 experiment of breeding two hybrids, there occurs the oppor- 

 tunity for one albino germ-cell from each of the two sexes 

 to unite and produce an albino young. The mathematical 

 chances of such an occurrence may be expressed in an alge- 

 braic formula in which D represents dominant germ-cells 

 (half the total number) and R represents recessive germ- 

 cells (half the total number). A D-cell may unite with 

 another Z>-cell, or it may unite with an It-cell. Similarly an 

 R-cell may unite with an A'-cell or with a D-cell, thus : 



