1 The Birth of Science - MSRI

140 5. Medicine and Other Empirical Sciences
but each springs forth in its way and preserves
its distinctive features, by a firm law of nature. 67
We see that the notion of a species as a pool of individuals capable of interbreeding,
often considered modern, must have been clear in Lucretius’
source, for he transmits it in an intelligible form. Elsewhere he expresses
the ideas about natural selection that we saw in the Aristotelian passage, 68
and he even says
For time changes the nature of all in the world
and everything must go from one state to another,
nor does anything remain like itself: all moves,
all is changed by nature, which imposes transformations.
One thing rots with time and languishes, weakened;
another, once insignificant, gathers strength. . . .
Many races of animals must have perished, must not
have been able to propagate by procreation.
Those you see now enjoying the life-giving air
Owe to their wiles or their strength or swiftness
Their preservation since the beginning of time. 69
What Lucretius knew about biology is not terribly clear: shortly before
this passage he discusses spontaneous generation (as having been once
very common, now characteristic of animal organisms that arise from putrefaction)
70 while elsewhere he seems to maintain that specific seeds are
needed for the generation of any living being. 71 The passage about extinct
animals that were not able to procreate might seem to imply abiogene- page 183
sis, but Lucretius is talking of extinct species, not individuals, and then he
states that the extant races (saecla) were preserved by their own fitness for
survival. Thus he does not have abiogenesis in mind in this case. That
in Lucretius’ sources natural selection operated by the gradual change of
67 Lucretius, De rerum natura, V, 916–924.
68 Lucretius, De rerum natura, IV, 823–842.
69 Lucretius, De rerum natura, V, 828–833, 855–859. In the intervening lines (omitted), Lucretius
talks about monstrous beasts of a distant past.
70 A belief in the spontaneous generation of animals was espoused by Aristotle (De generatione
animalium, 715a–b) and others.
71 Lucretius, De rerum natura, I, 159–207. The idea that animals cannot arise spontaneously
but must be born from other animals is generally considered modern and attributed to the
seventeenth-century physician Francesco Redi. But Alexander Polyhistor ascribes the same belief
to the Pythagoreans (Diogenes Laertius, Vitae philosophorum, VIII, 28). Redi, who devotes the first
pages of his Esperienze intorno alla generazione degl’insetti to an examination of the opinions held by
the “Ancients” on the subject, does not mention the Polyhistor passage reported by Diogenes; he
prefers to cite Diogenes when the latter quotes the opposite opinion that in bygone eras humans
were spontaneously generated ([Redi], p. 76; the English translation by Bigelow is incomplete).
Revision: 1.9 Date: 2002/09/14 19:12:01