3i8 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



cool temperature the flowers which develop later on will be normal 

 red in color. Thus it cannot be said that this primula inherits either 

 red or white flowers. What it really inherits is ability to react in 

 certain ways under the influence of temperature. 



c) Food aitd fertility. It is well known that the kind of food 

 supplied to the larvae of bees determines whether the females shall be 

 fertile (queens) or infertile (workers). The striking difi'erences in 

 structure and instincts of the two classes of females are all conditioned 

 by the food provided for the larvae. Each larva inherited the capacity 

 to react in either way according to the stimulus received. 



d) Moisture and structure. Morgan reports a variety of the 

 pomace fly, Drosophila ampelophila, with abnormal abdomen; "the 

 normal black bands of the abdomen are broken and irregular or even 

 entirely absent. In flies reared on moist food the abnormality is 

 extreme; but even in the same culture the flies that continue to 

 hatch become less and less abnormal as the culture becomes more 

 dry and the food scarce, until finally the flies that emerge later cannot 

 be told from normal flies. If the culture is kept well fed (and moist) 

 the change does not occur, but if the flies are reared on dry food they 

 are normal from the beginning." 



3. Environment may cause new heritable characters. — As yet 

 there is a dearth of evidence which can be accepted as scientific proof 

 that external stimuli actually cause germinal variations. At the same 

 time there is an abundance of data which falls into the class of circum- 

 stantial evidence in favor of such a doctrine. Moreover, there are 

 a few cases in which new heritable characters have been artificially 

 produced by carefully controlled external stimuli. Hence some 

 germinal variations are apparently caused by known environmental 

 conditions and we are justified in recognizing this third category of 

 developmental differences due to environmental effects. 



Considerable evidence of permanent changes in both morphologi- 

 cal and physiological characters has been secured from experiments 

 with the culture of bacteria and yeast, in unusual culture media, in 

 the presence of toxic solutions, or under extreme temperature condi- 

 tions. The significant results of four investigators who worked 

 independently, Hansen, Barber, Wolf, and Jordan, have been reviewed 

 and discussed in regard to their bearing on genetic theory by Cole 

 and Wright. The four investigators mentioned above used refined 

 methods and three of them began by isolating a single organism from 

 whose progeny they obtained distinct strains or biotypes which 



