CHAPTER XIV 

 THE BIOLOGICAL BACKGROUND OF GENETICS 



RACES AND INDIVIDUALS 



Evolution is racial change. In order that a race may change, the 

 individuals comprising the race must in large numbers exhibit changed 

 conditions. One or a few individuals changing in some peculiar way 

 may not at all affect the status of the species as a whole. It is well, 

 then, to remember that true evolutionary changes are mass changes 

 involving whole populations through successive generations. A race 

 or a species is to be looked upon as a vast unit continuous in time and 

 space. The members of a race are all descended from common an- 

 cestors and are interbreeding in all sorts of ways. If one should work 

 out a diagram of the genetic relationships of the members of a large 

 species, such a diagram of connecting lines of relationship would con- 

 stitute an almost solid mass of interlacing lines. An intricately inter- 

 connected group of individuals constituting a race is then a true evolu- 

 tionary unit. Just as cells are the structural units of life, so races are 

 the evolutionary units of life. Let us now briefly examine the consti- 

 tution of a race. 



At any given period of time a race consists of a large group of in- 

 dividuals. Some of these are related as parents and offspring, others 

 as brothers and sisters, others as cousins of varying grades; and all 

 doubtless trace back to some common ancestors a score of generations 

 ago. The individuals constituting the race at one period are not at 

 all the same individuals that constituted that race a few years pre- 

 viously, and none of them will be represented in that race a few years 

 hence. Individuals, then, are temporary components of the race; but 

 the race itself is permanent and relatively constant, though slowly 

 changing as a whole. 



It is our problem to determine the relation of the individual to the 

 race and to account for both the relative constancy and the slow change 

 of the race made up as it is of such mortal units as individuals. More 

 specifically, we must study the make-up of the individual and learn 

 how replacements are made when individuals wear out. 



THE CELLULAR MAKE-UP OF INDIVIDUALS 



All living things, except possibly the filterable viruses and bac- 

 teriophages, are made up of vital units known as cells. Many of the 



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