142 GENETICS 



white; another part causes the body to be yellow; and 

 these parts can be separated. In the original mother, one 

 part of the chromosome caused the eyes to be red, another 

 part caused the body to be yellow; and these parts also can 

 be separated. Different parts of the chromosome affect dif- 

 ferent characteristics. 



In a later chapter it will be shown that the chromosome 

 may by means of radiation be broken into two or more visi- 

 ble pieces, and that the different pieces produce the diverse 

 characters that are ordinarily connected with the single 

 chromosome. The relations shown In the exchange of parts 

 by two chromosomes, described above, are fully confirmed 

 by the results of physically breaking the chromosome Into 

 pieces. 



The separable parts of the chromosome, with diverse func- 

 tions, are known as genes. It Is Important to observe that 

 the existence of the genes, as separable parts of the visible 

 chromosomes, is definitely proved by experimentation. 

 Genes are not mere hypothetical units with imaginary prop- 

 erties, as is sometimes asserted. Genes are separable parts 

 of the chromosomes, having diverse effects on develop- 

 ment; their existence Is solidly demonstrated, and can be 

 verified by anyone who will take the necessary trouble. 

 There Is of course room for theorizing. In the present state 

 of our knowledge, as to the precise nature and action of 

 genes, but this does not touch the fact of their existence and 

 effects. 



How many separable parts are there In the chromosome? 

 That Is, how many genes are there? And how are they ar- 

 ranged; how are the parts put together to form the chro- 

 mosome? And how Is their number and arrangement dis- 

 covered? 



These questions are answered through an examination of 

 the processes of exchange of parts of the chromosomes, in 

 the case of different sets of characters. The evidence Is some- 



