THEORETICAL BIOLOGY 



475 



buds, which develop non-sexually ; the seeds on the other hand 

 usually give rise to fruits like the wild varieties from which 

 they originally arose. Among animals we may note the tend- 

 ency of domestic pigeons to revert to the ancestral blue rock 

 pigeon ; horses occasionally exhibit stripes like those of their 

 wild ancestors. 



The great advances which have recently been made in our 

 knowledge of heredity have been through the application oJ 

 Mendel's law, so named from its discoverer. The details of 

 this law are too complicated for elementary treatment, but the 

 fundamental principle is simple, — it is the law of the "purity 

 of the germ-cells." A simple concrete example will serve in 

 illustration. Let us take two animals of the same species, 

 which differ in some one striking characteristic, such as color in 

 the gray mouse and the white, albino mouse ; crossed with one 

 another they will produce offspring all of which will be gray, 

 hence the gray characteristic is said to be "dominant," the 

 white, "recessive." Now the germ-cells which these gray mice 

 of the second generation produce will bear the gray character- 

 istic or the white, and the two in equal numbers, but no germ- 

 cell will have both characteristics; this is the principle of the 

 purity of the germ-cells and we see its effect in the next gen- 

 eration, for the gray mice of the second generation, when bred 

 with one another, will give rise to the third generation which 

 will consist of both gray mice and white mice in the proportion 

 of three to one. These white mice, if bred together, will pro- 

 duce only white offspring for all subsequent generations. These 

 gray mice, although all apparently alike, are really of two kinds ; 

 one third will produce only gray mice forever, but the other 

 two thirds are precisely like the gray mice of the second gen- 

 eration, and like them will produce both gray and white off- 

 spring in the proportion of three to one. The reason for this 

 simple. The gray mice of the second generation possess germ- 

 cells, half of which have gray characteristics and half white, — 

 to represent it graphically, G + W; the crossing of two such 

 individuals affords the following possible combinations, GG + 

 2GIV+WIV. The combination GG will naturally be gray 

 and can produce only gray offspring; the same law is true for 

 the combination WW; but in the combination GW, the 1! is 



