GENETICS 257 



ago a white-eyed fruitfly appeared in cultures comprising thousands of 

 individuals, every other fly in which was red-eyed. Furthermore, every 

 ancestor of this white-eyed fly for scores of generations had had red eyes. 

 New characters appearing suddenly in this manner are called mutations. 

 The common evening primrose Oenothera is every year producing plants 

 unlike their parents. There are those who regard these new characters 

 arising in the evening primrose as due to recombinations of genes already 

 present; but even if their view in certain specific cases should prove 

 correct, it is hardly to be doubted that the differences arose in the first 

 place as mutations. Scores of other examples of supposed mutations 

 could be cited, though the evidence that they are mutations is not in 

 every case so clear. Variations of this kind are strictly inherited. In- 

 deed, the requirement of inheritance is part of the definition of the word 

 mutation. 



The nature of mutations can only be surmised. Since they are 

 inherited new features, one naturally thinks of them as due to modifica- 

 tion of the genes. If the genes are complex chemical substances, as 

 suggested above, changes in their structure due to unusual chemical 

 reactions should not occasion surprise. But further discussion of such 

 irregularities must be deferred to the chapter on evolution. 



Certain kinds of variation are due to another cause, namely, the 

 environment. It is seldom that two organisms occupy exactly the same 

 kind of environment Points in the same pond are unequal in available 

 food, in amounts of dissolved gases, in temperature, in light, in predatory 

 animals. The eggs from which animals hatch are unequally provided 

 with yolk, and this is essentially a condition of the young animal's 

 environment. Differences in parental care after birth or hatching may 

 produce differences in the adult. Such effects of the environment, how- 

 ever, are probably not in any case among higher organisms, or at most 

 only rarely, inherited. With them the geneticist has little to do, except 

 to guard against being deceived by them. 



Genetics and Evolution. — The preceding discussions of heredity and 

 its mechanism, and of variation, will readily indicate how closely the 

 subject of genetics is bound up with the subject of evolution. It is 

 obvious to even the most casual reader that if changes of a permanent 

 or nearly permanent nature are produced in organisms, these changes are 

 properly the subject matter of both genetics and evolution. This close 

 connection has led to a broad conception of genetics which is nearly 

 equivalent to that of evolution itself, and, as stated in the opening para- 

 graphs of this chapter, such broad definitions of genetics have been 

 formulated and used. It is not proposed, however, to enter into a further 

 discussion of genetics in relation to evolution until in a later chapter 

 evidences of evolution from fields other than experimental breeding can 

 be collected and described. 



17 



