THE GREAT RELAY RACE 457 



this agrees in general with the fact of equal contributions to the 

 following generation of chromosomes from each parent. It has been 

 repeatedly shown by experiment, as, for example, with the eggs of 

 sea-urchins, that when more than one sperm enters and fertilizes an 

 egg, thus involving the presence of an atypical number of chromo- 

 somes, the resulting larvae are monstrous, or at least abnormal, and 

 do not long survive. Evidently this is a case of too much father! 

 The conviction of the responsibility of chromosomes in heredity is 

 further strengthened by a very large number of remarkably ingen- 

 ious investigations made in the last twenty years, centering about 

 the occasional abnormal behavior of chromosomes during the matura- 

 tion divisions, particularly with the much-studied banana-fly Droso- 

 phila, maize, and the jimson-weed Datura. It is not possible in this 

 limited summary to do more than to call attention to this brilliant 

 and complicated work, which goes far in confirming the importance 

 of chromosomes in heredity. It is earnestly hoped, nevertheless, 

 that the reader may eventually have the opportunity to explore this 

 fairyland of fact. Although it involves a somewhat discouraging 

 array of strange technical terms, such as non-disjunction, transloca- 

 tion, coincidence, inversion, duplication, deficiency, deletion, interfer- 

 ence, and ploidy, it turns out that the terms used are not at all 

 formidable upon closer acquaintance. 



Genes 



Although the chromosomes of the male and female germ cells 

 unite to build the "imponderably small" bridge over which the 

 hereditary load passes from one generation to another, they are not 

 in themselves the actual units of heredity. These ultimate bearers 

 of inheritance, which are borne by the chromosomes, are known as 

 genes, a name given them by the Danish botanist Johannsen (1859- 

 1927). Dr. W. E. Castle has defined a gene as "the smallest part of 

 chromatin capable of varying by itself." In other words, genes are 

 the ultimate invisible hereditary units and as such form the essential 

 subject matter of genetics. 



That no one has ever surely seen genes under the microscope does 

 not lessen the fact of their reality. Like the atoms of the chemist and 

 the electrons of the physicist, of whose reality there is no doubt, they 

 are too small to be seen by any means at present at our disposal. 

 We know next to nothing about the structure and chemical composi- 

 tion of these ultimate hereditary units ; nevertheless, we already know 



