GENES AND MANKIND — BLIVEN 297 



and among these descendants certain types of mutation will appear 

 lin definite percentages which the scientists can now predict in 

 advance. They can tell you before the baby Drosophilae are born 

 (when they are considered in sufficient quantities) about how many 

 will have dwarfed wings, or white eyes resulting from lack of 

 pigmentation, or other abnormalities. If mankind has ever come 

 any nearer than this to usurping the privileges of the Deity, I do not 

 know where. 



These changes in characteristics which the scientists create in the 

 laboratory will breed true for all time in the future unless affected 

 by additional mutation later on. Ten thousand years from now, if 

 these DrosopMlae were to continue having descendants that long, 

 their remote offspring would still have white eyes or dwarf wings, 

 because the genes creating the normal characteristics, colored eyes, or 

 large wings, are lacking or altered. To illustrate this with a more 

 familiar laboratory animal : you could cut off the tails of 1,000 gen- 

 erations of mice and the one-thousand-and-first would have tails as 

 long as those of the original ancestors. But destroy the chief long- 

 tail-producing genes in a single pair of mice, and their descendants, 

 if inbred and selected for taillessness until they are pure for this 

 characteristic, will be tailless forever more. 



The action of the genes must certainly be one of the most interest- 

 ing mechanisms in the realm of nature. It is hard to believe, yet it 

 is increasingly being proved, that almost every hereditary char- 

 acteristic of the individual is to be found packed away in these 

 microscopic particles. The genes, acting together, seem literally to 

 plan the growing organism. Science does not yet know just how 

 this is done, or the relation among the genes, the mysterious and 

 miraculous glands of internal secretion, and the tissues. It is 

 assumed that the genes control the shape that growth takes from 

 the beginning of the new life. There are embryonic tissues, called 

 by the geneticists "organizers," which evoke specific growth-reactions 

 in other embryonic tissues. In the human being each of the glands 

 of internal secretion — the pituitary, the thyroids, parathyroids, 

 pancreas, adrenals, and gonads — secretes one or more hormones which, 

 poured into the blood stream, keep the body functioning. Most of 

 them, indeed, are vitally necessary for the maintenance of life itself. 



It staggers the imagination to think of the genes at work 

 through the months and years creating every organ of the complex 

 electrochemical-physical machine for living that is the body. Either 

 directly, or indirectly through the endocrine glands, they bring each 

 of the organs to full usefulness at the proper time and in co-ordina- 

 tion with each other. Then think of the hundreds of thousands of 

 species of animals and plants, in each of which the genes follow the 



