296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



a bubble pipe in soapsuds and blow until a mass of tiny bubbles rose 

 above the surface of the water and almost overflowed the basin ? The 

 analogy is inaccurate, but the mental picture you will get is similar 

 to what happens in the process of cell division. Each of the 48 

 chromosomes in a cell divides by splitting lengthwise into two parts, 

 which separate, half of them going to the right, so to speak, and 

 the other half to the left. Then the nucleus begins to narrow in 

 the middle, into a dumbbell shape, with 48 chromosomes in each end. 

 The bar in the middle grows shut; the two ends break off, and we 

 have two cells each containing all the chromosomes of the original 

 one. This process is repeated; the two cells become four, the four 

 become eight, and so on, until their number rises into billions. 



The almost unbelievable work that has recently been done is to 

 identify the genes which create certain characteristics of the living 

 creature. With enormous patience and countless hours of weary 

 industry, the scientists have tracked down the characteristics 

 associated with individual genes. (This had been worked out in 

 theory some years before it was verified under the microscope 

 through experiments in artificial mutation.) In the case of the 

 jimson weed, for example, about five hundred new genes have been 

 discovered, of which more than seventy have been located in par- 

 ticular chromosomes. The scientist can put his finger on the 

 chromosome chart of the jimson weed and can say: "At this point 

 are found the gene or genes that cause the plant to have a rough 

 skin, or to carry tufts, or to be an albino." In the banana fly, simi- 

 larly, the scientists know where in the chromosome lie the genes 

 associated with certain definite characters such as the color of the 

 eye, the size of the wings, bands on the abdomen, and so on. More 

 than 500 genes in Drosophila have been definitely located. 



A few years ago it was discovered that carefully controlled bom- 

 bardment with X-rays, neutrons, or other radiation would affect the 

 genes. Sometimes they are altered or destroyed, sometimes the 

 arrangement of the sections of the chromosomes is changed. In a 

 state of nature, once in many times the genes are spontaneously 

 changed by extreme heat or cold, by old age, or for other reasons. 

 When the organism is subjected to X-rays, the process of change is 

 enormously speeded up. Large numbers of mutations can be 

 detected in the short space of two generations, and the scientists are 

 perhaps beginning to see the mechanism of evolution in operation 

 before their very eyes. 



The almost incredible fact is that the experts now know in 

 advance in a general way what will happen when Drosophila is sub- 

 jected to a moderately severe dose of X-ray. Perhaps half the flies 

 will be killed. Among the survivors some will have descendants, 



