An Introduction to a Biology 



he must know two things : he must know what is the par- 

 ticular improvement he wants to effect ; and this, I assume, 

 he does know. He must also be familiar with Mendel's rules ; 

 and this, I assume, he does not know, or he would not be 

 reading this article, which is written for those who are not 

 familiar with Mendel's work or with the rules which have 

 been based on that work. 



First of all, we will become familiar with the essential 

 facts discovered by Mendel. 



The simplest case, and one that tells us all we want to 

 know about Mendel's results, is that of a cross between a 

 tall and a dwarf pea. The practical difference between a 

 tall and a dwarf pea is that tall peas have to be staked and 

 grown in rows not closer than 6 feet apart, whilst the dwarf 

 peas do not require staking and can be sown in rows 2 feet 

 apart. The stumpiness of the dwarf pea is due to the short- 

 ness of the divisions of the stem between the joints where 

 the leaves are given off. These divisions in the tall pea are 

 more than twice as long as they are in the dwarf. 



Mendel crossed a tall with a dwarf pea, and found that 

 the resulting hybrid was in every case tall. It made no 

 difference whether the tall was used as the male parent and 

 the dwarf as the female, or the dwarf as the male and 

 the tall as the female. The resulting hybrids constitute 

 the first hybrid generation, as shown in Fig. I. 1 Seed is 

 saved from these hybrids of the first hybrid generation, 

 which have been allowed to self-fertilise. This seed, when 

 sown, produces a generation, called the second hybrid genera- 

 tion, which consists of three tails and one dwarf amongst 

 every four plants, on the average ; or 75 per cent, tall and 

 25 per cent, dwarf plants. These plants of the second hybrid 

 generation are again allowed to self-fertilise, and, when they 

 are ripe, seed is saved from them, This seed, when sown, 

 produces the third hybrid generation. A look at Fig. 1 shows 

 that the dwarfs of the second hybrid generation breed true 

 1 Vide infra, p. 265. 



263 



