298 The Living Plant 



but grasp it clearly at the start, — namely, that it applies only to 

 single individual characters, never to large collections of them, 

 and much less to the whole aggregate of characters displayed by 

 each parent. Each of the many characters transmitted by parents 

 to offspring conforms to this general principle, but they are 

 transmitted to no two of the offspring in the same combinations. 

 The offspring are thus like the different patterns displayed by the 

 same pieces of colored glass in the turning of a kaleidoscope; and 

 this is very well exemplified in the familiar cases among mankind, 

 where the characteristics of two parents reappear in the most 

 different combinations in their children. Nevertheless, in self- 

 fertilized races of plants it is possible to fix permanently certain 

 combinations of characters by breeding out their opposites, and 

 then these combinations repeat themselves with the greatest 

 fidelity. Such combinations we shall meet again in our chapter on 

 evolution under the name of genotypes. 



Finally, we consider for a moment the explanation of the re- 

 markable mathematical arrangement revealed by Mendel's Law. 

 In the first place it is plain that each definite character of the 

 adult individual is represented by some kind of determiner in the 

 germ cells (i. e. egg-cells and sperm cells), and that any individual 

 is a mosaic of cliaracters of which the germ cells (or, rather, their 

 chromosomes), are (!ollections of the corresponding determiners. 

 Also, the facts show that, in harmony with the behavior of 

 the chromosomes in cell-division and fertilization, while every 

 body cell of each individual contains two determiners for each 

 character (one derived from each parent), every germ cell, because 

 of the reduction division earlier mentioned (page 285), contains 

 only one or the other of these determiners and never both, a fact 

 expressed in the phrase ''purity of the germ cells." Thus in our 

 Peas, earlier instanced, each of the male cells, and also each of the 

 female cells, contain a determiner for either a yellow or a green 

 cotyledon, but never both. Now if large numbers of such male and 

 female germ cells are allowed to come together at hap-hazard, as in 



