190 Summary 



then there is the sieve of the web of life with its chang- 

 ing pattern, becoming on the whole more and more 

 subtle, and therefore more discriminative. Darwin's 

 theory of selection does not pretend to account for 

 the origin of variations or mutations ; but as a theory 

 of sifting it works well. 



16. Heredity is the relation of organic continuity 

 between successive generations. It has become the 

 subject of precise experimental inquiry. We know 

 why like tends to beget like and how the contrasted 

 "unit characters" or Mendelian characters of two 

 parents will be distributed in their offspring. There 

 is a rapidly increasing knowledge of the topography 

 of the hereditary factors or "genes" in the nuclear 

 rods (or chromosomes) of the germ-cells! It is pos- 

 sible to predict with accuracy what the result of 

 certain crossings will be in the first and second filial 

 generations, and it is possible to use Mendelism as 

 a method of transferring advantageous characters 

 from one variety to a neighboring one. 



17. Each of the characters of the individual or- 

 ganism is the resultant of the vital cooperation of the 

 hereditary "nature" and the appropriate "nurture" 

 — meaning by nurture the whole complex of environ- 

 mental, nutritional, and functional influences to 

 which the developing organism is susceptible. A 

 peculiarity in the nurture may induce a definite 

 change in the organism. Such changes are called 

 "somatic modifications" or "acquired characters." 

 They are of very frequent occurrence and they often 

 mean much to the individual. The important question 



