NATURE OF PLANTS 127 



is evident that they represent pure stock in which the black and 

 yellow unit characters have been entirely separated. The seeds 

 that were a mixture of black and yellow produce when crossed 

 among themselves in the third generation the same ratio of 

 dominants, recessive and mixtures as appeared in the second 

 generations, namely, one-fourth pure black, two-fourths black 

 and yellow with the black dominant, and one fourth pure yellow. 

 So we see that each generation after the first is characterized 

 by the appearance of a rather definite proportion of offspring 

 with pure dominant or pure recessive characters, and with mixed 

 characters. This ratio of variation is known as Mendel's Law. 

 Plant and animal breeders rely upon these separations in pro- 

 ducing new forms and by careful breeding they are able at will 

 to add to or remove from the offspring one or more of the 

 parental unit characters. In some cases this is possible with 

 such certainty that horticulturists have advertised new forms, 

 describing their characters and qualities, before they have been 

 bred. How applicable Mendel's law is to all cases of crossing 

 has not been determined. In the case cited above where only 

 two unit characters were involved there can be but three combi- 

 nations of them, i. e., pure black, pure white and a mixture of 

 black and white. It is manifest where many unit characters 

 appear that there will result a re-combination of these characters 

 in the offspring that is complicated and difficult to interpret. 

 When however the number of unit characters that appear in any 

 plant or animal are definitely known is is probable that the varia- 

 tion of the offspring can be definitely forecast. 



It must be borne in mind that the variations referred to above 

 do not result in the formation of new characters but in the re- 

 combination of characters already existing in the parents. As 

 to the actual origin of new characters there is no generally ac- 

 cepted explanation. On the other hand there is a widely preva- 

 lent view that insignificant departures may appear in any form 

 owing to some circumstance of environment and that these de- 

 partures may gradually become intensified so that after a long 

 lapse of time they are sufficiently pronounced as to warrant the 

 recognition of the forms possessing these characters as distinct 



