THE GROWTH OF BIOLOGICAL IDEAS 161 



that certain genes behave under certain circumstances as 

 though they were linked to others in heredity. Soon it be- 

 came apparent that the number of groups of linked genes is 

 the same as the number of different chromosomes (only four 

 in each cell, in Drosophila). Morgan and his collaborators 

 were soon able to say which chromosome was concerned with 

 the inheritance of which group of linked genes and, further, 

 in what order the genes were arranged along each chromo- 

 some. They could say that at this point on a given chromo- 

 some was the gene that expresses itself most obviously by its 

 effect on the shape of the wings; here, farther along the same 

 chromosome, another affecting the size of the legs; farther 

 again, a gene affecting body color; and farther still, one af- 

 fecting^ the size of the winsjs; and so on for hundreds of other 



genes. 



The evidence for the arrangement of the genes in a certain 

 order along the chromosomes was entirely indirect. The 

 chromosomes looked more or less the same all along their 

 length; there were no little marks that might actually be the 

 genes. The complicated indirect evidence was obtained, like 

 Mendel's, from the counting of the numbers of individuals 

 showing various inherited characters in each generation, not 

 from a minute study of the chromosomes themselves. It was 

 not until the nineteen thirties that final ocular proof of the 

 chromosome theory of heredity was obtained. It became 

 known that some curious objects in certain cells of Drosophila 

 and other flies were nothing but gigantic chromosomes, about 

 one hundred times as long as normal ones. They are like 

 tapes with stainable marks across them. These marks are 

 something like the divisions on a measuring tape but differ 

 in that some are thick and some thin; and these thick and 

 thin marks follow one another in a resrular order. That resf- 

 ular order is the same in very nearly all the corresponding 

 chromosomes in the cells of all the flies of the same species,— 

 very nearly, but not quite— and the exception gave the clue 

 to a most important discovery. A few peculiar specimens of 

 Drosophila were known, in which the ordinary indirect evi- 



