HEREDITY 267 



change — though the causes may be obscure — that environment 

 also changes; and that either or both kinds of changes may affect 

 development over a long period of time. Many experiments, 

 some of which are described in the following paragraphs show 

 that the genes can be affected by external causes. 



Direct Action of External Factors on Germ Cells. — Stockard 

 and Papanicolaou have succeeded in producing defects in the 

 eyes and skeleton of the limbs in guinea pigs whose parents had 

 been subjected to the inhalation of alcohol fumes. Since the 

 defects were inherited in succeeding generations without further 

 treatment with alcohol, it is clear that the germ cells of the 

 treated parents must have been affected by the alcohol. How- 

 ever, since the treated parents showed no ill effects from the 

 alcohol, the effect on their germ cells must have been direct 

 rather than through the somatic cells. There is in this case no 

 inheritance of acquired characters because the defects of the 

 offspring were not present in the somatic tissues of the treated 

 parents. 



Muller and others have produced hereditary effects by treating 

 fruit flies (Drosophila) with X rays. Here also the germ cells 

 were affected directly and the genes were sufficiently modified to 

 produce hereditary changes in the characters of the offspring. 



Parallel Induction. — It is conceivable that under certain 

 conditions both germ cells and somatic cells might be altered by 

 environmental or external causes and result in hereditary effects. 

 Such a result is known as -parallel induction, which may be 

 illustrated by the experiments of Guyer and Smith. These 

 investigators injected into the blood stream of fowls the pulped 

 lenses of rabbit eyes, for the purpose of producing an antilens 

 substance in the fowl's blood. The serum from such an immun- 

 ized fowl would therefore possess antilens properties. When 

 such antilens serum was injected into the blood vessels of preg- 

 nant rabbits, the offspring showed a number of eye defects of 

 which the most common were opaque lenses, small eyes, and 

 abnormally rotated eyes. The defects, though not confined to 

 the lens of the eye, were inherited through the female lines and 

 occasionally through the male. These results are regarded as 

 due in the first instance to the simultaneous effect of the antilens 

 serum, circulating in the blood of the pregnant mother, on the 

 eyes and germ cells of the developing embryos. 



