2 GENETICS 



work of life. Others, not actually feeble-minded, are dull. 

 Others are intelligent. Some have great abilities; they are 

 geniuses. Among those that are neither stupid nor geniuses, 

 some are fitted for one kind of work, some for other kinds. 

 There exist similar differences in senses, in tastes, in emo- 

 tions. 



Genetics deals with all such differences; it inquires as to 

 their source. It deals further with the great differences 

 between organisms, that make men classify them into dif- 

 ferent species, different genera, families and the like. 



Genetics deals also with similarities, for these are 

 merely degrees of difference. It inquires how it happens 

 that some individuals differ little, others much. Twins may 

 be so much alike that they are indistinguishable to the ob- 

 server. Other children of the same family resemble each 

 other, but less, as a rule, than do twins. Members of the 

 same race are more alike than members of different races. 

 The same sorts of differences and similarities are found in 

 other animals and in plants. They are what give variety 

 and individuality and interest to the world of men and of 

 other organisms, and they give rise to most of the problems 

 and difficulties of the living world. It is the task of genetics 

 to seek for the causes of the similarities and differences 

 among organisms. 



There are two main classes of differences between things. 

 On the one hand things may be diverse because they are 

 composed of different materials. An object made of iron is 

 very different from one made of glass, or of lead, or wood, 

 or water, or air. A physician's prescription has tremendously 

 differing effects, depending on the drugs it contains. 



On the other hand things may be diverse because they 

 have been treated differently, because of the different con- 

 ditions under which they exist or have existed. Water under 

 one set of conditions is solid ice, under others it is liquid 

 or vapor. A piece of iron treated in one way — shaped into 



