Morgan, Thomas Hunt, Experimental zoölogy

(New York : London :  The Macmillan Company ; Macmillan & Co., Ltd.,  1907.)

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CHAPTER IV

THE  INHERITED  EFFECTS   OF CHANGES INDUCED BY
EXTERNAL  FACTORS

It has been pointed out that in butterflies the changes brought
about by higher and lower temperatures give rise to forms that
resemble southern and northern varieties of the same butter¬
flies. The question at once arises whether species may not
have originated in this way.

Fischer found that when a dark moth, produced by cold,
was paired with another similar moth, the offsprings were also
dark. His experiments extended, however, only to the first
generation, and consequently the cold may have acted directly
on the germ-cells of the parents. Highly important as this
observation is in showing that the undeveloped germ-cells may
be affected in the same way as the somatic tissues of the pupa,
so that even under altered conditions the effect persists, yet the
result as it stands does not conclusively show that permanent
racial or specific changes have been produced in this way.
Were such forms bred for several generations at a warmer
temperature, it is possible that they would return again to their
original condition. The new type might persist only so long as,
or a little longer than, the external conditions are the same as
those that produced it. The change may represent only an
extreme fluctuating variation that has been caused by an ex¬
ternal factor. The results do not appear, from this point of
view, to belong to the kind of changes by which new species
are made. Nevertheless the question still remains an open
one as to whether changed external conditions may not at
times cause more permanent effects. Some observations of
Standfuss seem to show that such may be the case, although

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