GROWTH AND REPRODUCTION 157 



sive factors, while the other two contain equal amounts of both. 

 But in these latter, the recessive factor is prevented from showing 

 itself by the presence of the dominant factor. We have then some 

 individuals which are pure-breed, so that if mated with similar ones, 

 the progeny must be pure-breed. But those individuals which 

 contain dominant factors only cannot be distinguished from the 

 mixed ones, whereas those which show the recessive factor must be 

 purely recessive and can be depended upon to breed true. This 

 process is known as the segregation of pure gametes. The practical 

 object of breeding new races is thus to obtain the desired quality as 

 a recessive factor. Take the case of wheat. It is desired to obtain 

 a variety resistant to the attacks of the fungus known as " rust." 

 A race has been produced in which this character is recessive, and 

 hence can be bred true. 



It is not to be supposed that all inherited characters obey 

 Mendelian laws, and there are many complications in detail which 

 cannot be entered into here. 



The second aspect of heredity which requires notice is the 

 problem of the inheritance of characteristics acquired in the lifetime 

 of the parents themselves. It has been held by some that the 

 germ plasm conveys only those factors derived from distant 

 ancestry, and that it is in no way affected by what happens to the 

 individual organism which is its temporary host. There seems, 

 indeed, to be no satisfactory evidence of any kind of mutilation 

 happening to the parent being transmitted to the offspring, and the 

 germ plasm, as far as we know, is independent of nervous connec- 

 tion with the rest of the organism. At the same time, it cannot be 

 denied that it is accessible to chemical agents, and if such are pro- 

 duced by the various tissues, any alteration in them must have its 

 effect. It will be clear that the removal of any part which leaves 

 similar structures untouched cannot be expected to have any 

 chemical effect of the kind mentioned, while the removal of any 

 organ which is the only representative of its particular tissue is 

 usually followed by death. Thus the loss of a leg leaves similar 

 tissues intact, while that of the liver or suprarenals is fatal. That 

 the germ plasm is accessible to chemical influence is shown by the 

 experiments in which guinea-pigs were allowed to breathe the 

 vapour of alcohol. The offspring of the alcoholized animals were 

 deficient in strength and vitality, and often showed coarse abnor- 

 malities. The effects were transmitted through several generations, 

 even more marked in the later ones, although no exposure to 

 alcohol had been made since that of the original parents. No 

 changes in the sexual glands were visible under the microscope, 

 but the effects produced would be too subtle to be detected thus. 



Variation. Although the offspring are very like their parents, 



