Species

The Species Problem Arises 205

as a good test of species, and also the proposal to give up the notion of species.

Although there are no absolute criteria for judging species, Robson does think

that well-dened groups do occur in nature, and he rejects the “morphological,”

“genetic,” “physiological,” and “ecological” criteria as being either incomplete or

ill-dened tests of species rank.^79

The Mendelians: Morgan and Sturtevant

The famous Chicago geneticist Thomas Hunt Morgan was initially of the view that

species were nonexistent, arbitrary units devised for the convenience of taxonomists

(and hence is a conventionalist species denier at this stage). He held that, with Buffon,

only individual organisms existed:

We should always keep in mind the fact that the individual is the only reality with

which we have to deal, and that the arrangement of these into species, genera, families,

etc. is only a scheme invented by man for purposes of classication. Thus, there is no

such thing in nature as a species, except as a concept of forms more or less alike.^80

His view of species as types determined by convention never changed there-

after, although he did later indicate that we think of species in terms of their

adaptations.

He did allow that there was an enormous amount of variability in genes, though.

His students, in particular Alfred H. Sturtevant, had a stronger interest in species,

and Sturtevant held that there was a “wild-type” of each species from which vari-

ants were insignicant and that species actually differed in few genes.^81 H. J. Muller,

however, later adopted in full the neo-Darwinian account of the Modern Synthesis,

treating species as formed through hybrid infertility due to genetic variations such

as chromosomal rearrangements. Even in his Drosophila, the nature of species was

plastic and variable, and

[i]t becomes, then a matter of denition and of convenience, in any given series of

cases, just where we decide to draw the line above which two groups will be distin-

guished as separate species, and below which they are denoted subspecies or races,

since in nature there is no abrupt transition here.^82

Despite this, he said, a “well-knit” species is qualitatively different from individu-

als of two closely related species, and the word “species” denotes

at least to a rough approximation, how these groups stand in relation to that general

level at which ‘speciation’ takes place.^83

(^79) Robson 1928, 21f.
(^80) Morgan 1903, 33 quoted in Allen 1980, 359f.
(^81) Dobzhansky 1980.
(^82) Muller 1940, 252f.
(^83) Muller 1940, 254.