Species

The Nineteenth Century, a Period of Change 141

There are fixed limits beyond which the descendants from common parents can never

deviate from a common type. ... It is idle ... to dispute about the abstract possibility

of the conversion of one species into another, when there are known causes, so much

more active in their nature, which must always intervene and prevent the actual accom-

plishment of such conversions.^124

Mayr thinks this is an expression of Lyell’s essentialism: “each species had its
own specific essence and thus it was impossible that it could change or evolve. This,
for example, was the cornerstone of Lyell’s thought.” But was it really? Lyell seems
to be saying not that an essence is causing it to remain stable, but that a species is
held stable by interbreeding and “known causes” of infertility. There is typology,
to be sure, but overall, Lyell’s view is a causal one: again, we see here hints of a
generative notion of species. However, Mayr correctly notes that the Principles was
Darwin’s scientific “bible”^125 and that he devoted so much time in the Origin to refut-
ing special creation largely because of the challenges set by Lyell.^126

A-P DE CANDOLLE AND ASA GRAY:

THE BOTANICAL VIEW OF VARIATION

A third stream of thought in this period supposes that species are real and that so
also is variation from the type. This is primarily due to the famous family of Swiss
botanists, the de Candolles, in particular the elder, Augustin-Pyramus.^127 A-P de
Candolle stressed the variation of living things, and defined species as

... the collection of all the individuals who resemble one another more than they

resemble others; who are able, by reciprocal fecundation, to produce fertile individu-

als; and who reproduce by generation, such kind as one may by analogy suppose that

all came down originally from one single individual.^128

Elsewhere, in the Elementary theory, he writes

By Species (species), we understand a number of plants, which agree with one another

in invariable marks.^129

So, for de Candolle species are groups of individual organisms. They are, in the tra-

dition of Ray, to both resemble one another and to generate progeny that are fertile and

resemble one another. As did Buffon and the older tradition, De Candolle treated varia-

tion as the effect of local environments and occasional hybridization. In this opinion,

he was followed closely by the great American botanist, Asa Gray, who was later to

become significant in the promotion of Darwinian theory in America against Agassiz.

As late as 1846, in a review of the Vestiges of Natural Creation, Gray declared that

(^124) From the 1835, 4th, edition, vol. II, p 433; the second sentence is not found in the first edition.
(^125) Mayr 1982, 406.
(^126) Mayr 1982, 407.
(^127) Ca ndolle 1819.
(^128) Quoted in Hunter Dupree 1968, 54.
(^129) From Candolle and Sprengel 1821, 98.