Species

178 Species

compiler W. S. Dallas, not of Darwin. David Williams notes that Dallas’ definition

of terms like homology differed from Darwin’s use of the term.^80

After the Origin

After the Origin, Darwin is able to publish some generalizations about species. He

does so in chapter 2 of the Descent of Man:

If we consider all the races of man as forming a single species, his range is enormous;

but some separate races, as the Americans and Polynesians, have very wide ranges. It

is a well-known law that widely-ranging species are much more variable than species

with restricted ranges; and the variability of man may with more truth be compared

with that of widely-ranging species, than with that of domesticated animals.^81

The “well-known law” is in part of Darwin’s own construction, as we have seen.
In the Variation of Plants and Animals under Domestication, published in 1868
and revised in 1875,^82 Darwin discusses how species arise in nature, and proposes
intersterility as a criterion of species-status.

... how, it may be asked, have species arisen in a state of nature? The differences

between natural varieties are slight; whereas the differences are considerable between

the species of the same genus, and great between the species of distinct genera. How

do these lesser differences become augmented into the greater difference? How do

varieties, or as I have called them, incipient species, become converted into true and

well-defined species?^83

Using domestic varieties as a guide to variation in nature, as he had in the Origin,

he notes

... that the sterility of distinct species when crossed, and of their hybrid progeny,

depends exclusively on the nature of their sexual elements, and not on any differences

in their structure or general constitution. ... That excellent observer, Gärtner, likewise

concluded that species when crossed are sterile owing to differences confined to their

reproductive systems^84

and in answer to the question of how it is that extremely different domesticated

varieties are “perfectly fertile” while closely allied species are not, he responds:

[p]assing over the fact that the amount of external difference between two species is no

sure guide to the degree of their mutual sterility, so that similar differences in the case

of varieties [of domestic animals and plants—JSW] would be no sure guide, we know

that with species the cause lies exclusively in differences in their sexual constitution.^85

(^80) Pers. comm.
(^81) Second edition, Da r win 1871, 112.
(^82) Da r win 1868, 1875; modern edition 1998.
(^83) Da r win 1868, Vol. I, 45.
(^84) Da r win 1868, Vol. II, 168f.
(^85) Da r win 1868, Vol. II, 172.