312 ANIMAL BIOLOGY 



Such are a few of the types of mutations in the nuclear complex 

 of the germ cells that we now know give rise to genetic variations. 

 Chromosomal aberrations afford new relations and proportions 

 of their constituent genes, whereas gene mutations actually deter- 

 mine the nature of the chromosomal elements themselves. In 

 many instances, probably the majority, mutations produce lethal 

 combinations in the gametes or zygotes — the altered hereditary 

 constitution renders development or survival impossible. In other 

 cases, individuals with the mutant characters become established 

 and produce offspring with these new characters and so supply the 

 material for descent with change. 



Finally, in passing, it should be mentioned that mutations may 

 also occur in somatic cells. Such somatic mutations give rise 

 to changes in the individual body which, of course, cannot be 

 transmitted by the germ cells, but may be perpetuated by vegeta- 

 tive reproduction. Thus many desirable types of fruit are produced 

 solely by tissue, descended from the original plants in which the 

 mutations occurred, that has been grafted on other plants. 



Important as is the recently acquired knowledge of some of the 

 nuclear changes at the basis of mutations, we still do not know 

 the fundamental factors underlying their origin. However, recent 

 experiments have afforded a most valuable clue. It has been found 

 possible to induce mutations in certain animals and plants by sub- 

 jecting their germ cells to unusual conditions — the most effective 

 so far employed being irradiations (X-rays, etc.) and certain tem- 

 perature changes. Thus since mutations can be induced by con- 

 trolled and measured external agents, the way seems to be open- 

 ing for an experimental attack on the problem of the origin of 

 mutations in nature that apparently is at the basis of organic 

 evolution. 



E. Nature and Nurture 



Even after making due allowance for the possibilities of genetic 

 change involved in mutations, the individual still may be considered 

 as a composite of very many characters which usually behave in 

 a definite way in inheritance. Expressed somewhat fancifully, 

 individuals may be regarded as temporary kaleidoscopic recom- 

 binations of the various genes belonging to the species ; the act of 

 reproduction, especially the maturation divisions, involving segre- 



