Gilbert: The Science of Genetics 



239 



In systematizing any body of facts 

 to form a science, it is first necessary to 

 detennine the underlying units out of 

 which the facts are made up. ..Alchemy 

 could not become chemistry until its 

 fundamental units were discovered and 

 identified. So the science of genetics 

 could not exist until the conception 

 was developed that the individual is 

 made up of smaller heritable units 

 called "unit characters." Thus we are 

 concerned with the inheritance of unit 

 characters and not with the individual 

 as a whole. This gives us a method of 

 analysis hitherto little employed in 

 biology. 



The observed phenomenon of varia- 

 tion is merely the manifestation of 

 different combinations of these unit 

 characters. Obviously, when individ- 

 uals are made up of many unit charac- 

 ters, say five hundred to a thousand, 

 the possible combinations of these units 

 are almost limitless and thus there 

 appears the variability which we ob- 

 serve. 



The external manifestation of a 

 plant or animal is. furthermore, little 

 criterion of its actual genetic composi- 

 tion. Animals may be the same in 

 appearance but differ widely in their 

 offspring. Gray rabbits, for example, 

 may be synthetically produced by 

 bringing together a nrmiber of entirely 

 different combinations of unit charac- 

 ters. Yet these gray rabbits look the 

 same. Every organism has its peculiar 

 formulae of body cells and germ cells, 

 or as genetists say, its own genotypic 

 and phenotypic formulae. 



This principle was recognized many 

 years ago by Jordan of France, who 

 believed that species should be classified 

 according to their method of transmis- 

 sion and not according to their exter- 

 nal appearance. Thus we have the so- 

 called "Jordan's Species." 



the question of "pure lines." 



Another problem which is puzzling 

 the genetist at present is the theory of 

 the pure line, projected by Johannsen. 

 This deals with another phase of the 

 unit character idea. If all organisms 

 are made up of unit characters, these 

 must exist as homogeneous combinations 

 or heterogeneous ones. The offspring 



of a self-fertilized plant having similar 

 unit characters is called a "pure line." 

 According to this theory, the process of 

 selection as we ordinarily consider it is 

 a process of isolating pure lines. When 

 this is done, according to Johannsen, no 

 further progress in changing the race 

 is possible unless a mutation or the 

 sudden manifestation of a new unit- 

 character appears. According to this 

 notion, selection is a process of isolation 

 and elimination and has no power to 

 cause advancement. This leads us to 

 consider another and greater problem, 

 perhaps the greatest of all — the cause 

 of variation and mutation. 



If evolution takes place by the ap- 

 pearance of new unit characters or 

 mutations — where do these come from? 

 and can they be controlled? This is 

 the eternal mystery. Experiments give 

 us little clue. Many experimental 

 attempts have been made to produce 

 mutations but all have failed. All 

 apparent successes have turned out to 

 be illusory, when subjected to rigid 

 tests. Most have proved to be not new 

 characters at all, but merely new com- 

 binations of old ones. Of course, we 

 can produce such new combinations, in 

 many cases, at will, but that is a far 

 different thing from actually producing 

 a new character. At present, our only 

 means for getting a new character is to 

 sit down and wait until it appears — • 

 whence or how we know not. Even 

 when it has appeared in this way, it is 

 quite as likely to disappear again, if it 

 proves disadvantageous to the individ- 

 ual carrying it. In such an event 

 natural selection eliminates it (by 

 destroying the individual carrying it) — 

 unless it should happen to be useful to 

 man. In the latter case, we artificially 

 preserve it by removing the individual 

 carrying it from competition with its 

 wild relatives, and putting it to our 

 own use. That is why domesticated 

 races of animals or plants differ from 

 their wild relatives. 



One of the greatest problems, even 

 yet, before the student of genetics is 

 that much discussed subject, the inheri- 

 tance of acquired characters. Are the 

 qualities which an organism receives 

 during its lifetime passed on to its off- 

 spring? If so, by what mechanism is 



