72 ADAPTATION AND PROGRESS 
The so-called Mendelian law, the outgrowth of years of experi- 
ment by Mendel in crossing garden peas of different varieties, 
and verified since by hundreds of experiments on various plants 
and animals by many botanists and zodlogists, is thus stated and 
illustrated by Professor Walter: ! 
When parents that are unlike with respect to any character are crossed, 
the progeny of the first generation will apparently be like one of the parents 
with respect to the character in question. The parent which impresses its 
character upon the offspring in this manner is called the dominant. When, 
however, the hybrid offspring of this first generation are in turn crossed with 
each other, they will produce a mixed progeny, 25 per cent of which will be 
like the dominant grandparent, 25 per cent like the other grandparent, and 
50 per cent like the parents resembling the dominant grandparent. 
The law is represented by the following figure which shows that 
in monohybrids the dominant character comes to the surface in 
the second generation in three out of four of the offspring, one 
of the three, however, being of pure breed, the other two being 
hybrids. 
D (DominanT) R (RECESSIVE) 
D(R) 
| | 
DD 2D(R) RR 
| | | 
i D ri 2D(R) RR RR 
| | | 
| | | | | | | 
DD DD DD 2D(R) RR RR RR 
There are three principles involved in this law: (1) The exist- 
ence of independent unit characters, (2) dominance, in cases where 
the parents differ in a unit character, and (3) segregation of the 
units contributed by the respective parents, this segregation being 
found among the gametes formed by the offspring.? 
1 Genetics, pp. 123 f. 
2 Castle, Heredity, p. 38; cf. Walter, op. cit., pp. 144, 145. 
