Maech 20, 1903.] 



SCIENCE. 



447 



were cross-pollinated in the summer of 1900 

 with pollen from angular-seeded plants, or 

 vice versa, and that an average of four seeds 

 formed in each pod. With the death of 

 the parent plants the old generation ended, 

 and the 100 seeds that matured in 1900 — 

 the year in which the cross was made — be- 

 gan the next generation; and these 100 

 seeds were hybrids. Now, all these 100 

 seeds were round. Roundness in this case 

 was 'dominant.' (Dominance pertaining to 

 the vegetative stage of the plant of course 

 would not appear until 1901, when the seeds 

 'grow.') These seeds are sown in the 

 spring of 1901. If each seed be supposed 

 to give rise to four seeds— or 400 in all— 

 this next generation of seeds (produced in 

 1901) wiU show 300 round and 100 angular 

 seeds. That is, the other seed-shape now 

 appears in one foui'th of all the progeny; 

 this character is said to have been 'reces- 

 sive' in the first hybrid generation. If 

 the 100 angular seeds, or recessives, are 

 sown in 1902 it will be found that all the 

 progeny will be angular-seeded or will 

 ' come true ' ; and this occurs in all succeed- 

 ing generations, providing no crossing takes 

 place. If the 300 round seeds, or domi- 

 nants, are sown in the spring of 1902, it will 

 be found that 100 of them produce domin- 

 ants only, and that 200 of them behave as 

 before— one fourth giving rise to recessives 

 and three fourths to dominants; and this 

 occurs in all succeeding generations, pro- 

 viding no crossing takes place. In other 

 words, the three fourths of dominants in 

 any generation are of two kinds, — one 

 third that produce only dominants and two- 

 thirds that are hybrids. That is, there is 

 constantly appearing from the hybrids one 

 fourth part that are recessives, one fourth 

 part that are constant dominants, and one 

 half part that are dominants to all appear- 

 ances, but which in the next generation 

 break up again into dominants and reces- 



sives. This one half part that breaks up 

 into the two characters consists of the true 

 hybrids; but they are hybrids only in 

 the sense that they hold each of the 

 two parental characteristics— roundness 

 and angularity — in their purity and not 

 as blends or intermediates; and these two 

 characteristics reappear in all succeeding 

 generations in a definite mathematical 

 ratio. Proportionally, these facts may be 

 expressed as follows: 



It will be seen that two thirds of the 

 dominants break up the following year 

 into one fourth constant dominants, one 

 fourth recessives, and one half that again 

 break up, the half that break up being the 

 hybrids. This formula for the hybrids 

 is Mendel's law. In words, it may be ex- 

 pressed as follows: Differentiating charac- 

 ters in plants reappear in their purity and 

 in mathematical regularity in the second 

 and succeeding generations of hybrid off- 

 spring of these plants; the mathematical 

 law is that each character separates in each 

 of these generations in one fourth of the 

 progeny and thereafter remains true. In 

 concise figures it is expressed as follows : 



ID:2DR:IR. 



ID and IjB come true, but DB breaks up 

 again into dominants and recessives in the 

 ratio of 3 to 1. 



Mendel found that this law holds more 

 or less for the other characters that he 



