THE GREATEST COINTRIBUTOK l() (iENETICS 



Around any decaying fruit may be found little red-eyed flics, about one-eighth of an inch 

 long, which are known to science under the name of Drosophila. They are so insignificant 

 that the housewife regards them as hardly even a i)est; but to a geneticist they are 

 perhaos the most important insect in the world, for their study has thrown more light 

 on heredity than has that of any other one animal. They are, in fact, practically an 

 ideal subject for study, since they stand confinement well, can be kept indefinitely 

 in a milk bottle with a little overripe banana, and in ten days a single pair of parents 

 will produce a generation of 200 or 300 young. Further, they have a number of well- 

 defined characters that can be followed in heredity, and the number of chromosomes 

 is conveniently small (four). Although geneticists have been searching for ten years 

 to find another equally satisfactory insect, they have failed. Photomicrograph by 

 John Howard Paine. (Fig. 7.) 



bring about a decrease in size or strength 

 which will likewise be inherited. This 

 theofy of the inheritance of the effects 

 of use or disuse — of the inheritance of 

 acquired characters, to use the cus- 

 tomary title — is associated principally 

 with the name of the French zoologist, 

 Lamarck. It was accepted by Darwin, 

 who made much use of it. It held 

 until a generation ago, when August 

 Weismann discredited it by an ajjpeal to 

 common sense. Today the theory has 

 few followers amongst trained investi- 

 gators, but it still has a ])oi)tilar follow- 

 ing that is widesjoread and vociferous. 



3. There is a third explanation of 

 evolution which has taken protean 

 forms. At one extreme it is little more 

 than a mystic sentiment to the effect 



14 



that evolution is the result of an inner 

 driving force. The earliest name of 

 prominence associated ^vith it is that of 

 Nageli; recently the geneticist, William 

 Bateson, has put forward a theory of 

 evolution of somewhat similar nature. 

 The nimierous theories of this type 

 may be collected under the title of "The 

 theory of the unfolding principle." 



NATURAL SELECTION 



4. Darwin and Wallace put forward 

 the last of the great historical specula- 

 tions al)out evt)ltUion in the well- 

 known theory of natural selection, and 

 the former brfmght together so much 

 evidence as to win almost universal sui)- 

 port from men of science. He ap- 

 pealed to chance variations as supply- 



