31 

 The Bridge of Life 



Between one generation of animals and the next there exists a tiny 

 protoplasmic bridge over which all of the inherited characteristics must 

 be passed. The tiny sperm of the male and the somewhat larger egg 

 of the female must contain the factors that account for all of the in- 

 herited characteristics that pass through the generations. It is no 

 matter of chance or environment that causes a female cat to give birth 

 to kittens rather than puppies or a hen's egg to hatch into baby chicks 

 rather than ducklings. There are genes within the reproductive cells 

 from which these animals are derived that cause them to develop into 

 something quite like their parents. Yet, this similarity is never com- 

 plete. Human children are never exact duplicates of their parents — 

 they show some characteristics found in one parent, some found in the 

 other, some which are intermediate between the two, and some that 

 are shown by neither parent. Children of the same parents will also 

 show differences among themselves, yet they will be more alike than 

 first cousins. First cousins, on the other hand, will show greater simi- 

 larities than nonrelated children of the same race, and children of the 

 same race will be more alike than children of different races. To go a 

 step further, children of different races certainly resemble one another 

 more than the offspring of different species of animals. These are 

 commonly observed facts which have their basis in the similarities and 

 differences of genes among these forms of life of differing relation- 

 ships. It will be our purpose in this chapter to learn how these genes 

 bridge the gap from one generation to the next. We will begin with 

 some of the early speculations on this interesting subject of genetics in 

 order that we may see how science has replaced pure speculation and 

 superstition with verifiable facts. 



Early Speculations on Heredity 



The famous Greek philosopher, Aristotle, who lived several hundred 

 years before the time of Christ, gives us one of the earliest recorded 

 theories of heredity. He suggested that the semen of a man was pro- 

 ducd from his blood ; in fact, he thought of it as highly purified blood 



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