THE DOCTRINE OF EVOLUTION 113 



Gregor Mendel was an Austrian monk, who experimented 

 with plants in his garden, and in 1865 communicated to the 

 Society of Naturalists at Briinn the substance of what has 

 since been described as " the greatest discovery in biology 

 since Darwin." As the result of later experiments the law 

 was rediscovered by Bateson. In our own country Professor 

 Castle, of Harvard University, has made observations on many 

 successive generations of guinea-pigs, mice, and rabbits. The 

 substance of most of the following statements has been taken 

 from his studies. 



Mendel's law asserts that when mating occurs between two 

 animals differing in some character, the offspring (hybrids) 

 will frequently exhibit the characters of one parent only. 

 A particular character exhibited in that way is said to be 

 " dominant." If albino rabbits (white, with pink eyes) are 

 crossed with gray rabbits, all the offspring are gray, that color 

 being dominant. The character (in this case, whiteness) which 

 is not seen in the immediate offspring is said to be "recessive." 

 That the albinism (white character) is really present in the 

 second generation, although invisible, may be demonstrated 

 by permitting pairs of these hybrid rabbits to breed. The 

 principle may be called " the law of dominance." It does not 

 seem to be of universal application, as sometimes the young 

 have a character of their own. Thus, if a lop-eared rabbit is 

 bred with a short-eared rabbit, the young will have ears of 

 an intermediate length. There is no way of knowing what 

 the character of the young will be ; it can only be determined 

 by experiment. When once determined, however, the char- 

 acter is always the same for every hybrid, provided the par- 

 ents are of pure breeds. 



The essential part of Mendel's discovery is this : that 

 the hybrids resulting from the crossing of animals, such as 

 white and gray rabbits already spoken of, whatever their own 

 character, will produce ripe germ-cells (reproductive cells, 



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