nominal species have been described for each one that is now considered
distinct."[20]

[Footnote 19: "Mendel's Principles of Heredity," p. 284, 1909.]

[Footnote 20: "Science Sketches," p. 99.]

And again:

"Thus the common Channel Catfish of our rivers has been described as a
new species not less than _twenty-five times_, on account of
differences real or imaginary, but comparatively trifling in value."[21]

[Footnote 21: "Science Sketches," p. 96.]

Perhaps the reader will tolerate another somewhat long quotation because
of the light which it sheds on this whole problem.

"Some years ago we had a parasite of a very destructive aphid down in
our books as _Lysiphlebus tritici_. In carrying out our investigations
it became necessary to find out whether this parasite had more than a
single host insect, and whether it could develop in more than one
species of aphid. To this end, recently emerged males and females were
allowed to pair, after which the female oviposited in several species of
aphids. Both parents were then killed and preserved and all of their
progeny not used in further experiments were also preserved, and thus
entire broods or families were kept together. In this way females were
reared out of one host species and allowed to oviposit in others, until,
often after several hosts had been employed, it would be bred back into
the species whence it first originated. In all cases the host was reared