Other animals have marks of having in a long process of time undergone changes in some parts of their bodies, which may have been effected to accommodate them to new ways of procuring their food. The existence of teats on the breasts of male animals, and which are generally replete with a thin kind of milk at their nativity, is a wonderful instance of this kind. Perhaps all the productions of nature are in their progress to greater perfection? an idea countenanced by the modern discoveries and deductions concerning the progressive formation of the solid parts of the terraqueous globe, and consonant to the dignity of the Creator of all things. (The Temple of Nature, 54)
A great want of one part of the animal world has consisted in the desire of the exclusive possession of the females; and these have acquired weapons to bombard each other for this purpose, as the very thick, shield-like, horny skin on the shoulder of the boar is a defense only against animals of his own species, who strike obliquely upwards, nor are his tushes for other purposes, except to defend himself, as he is not naturally a carnivorous animal. So the horns of the stag are not sharp to offend his adversary, but are branched for the purpose of parrying or receiving the thrusts of horns similar to his own, and have therefore been formed for the purpose of combating other stags for the exclusive possession of the females; who are observed, like the ladies in the times of chivalry, to attend the car of the victor. (Zoonomia, 1794, revised 1801, I:503)
Whence it appears, that many of those vegetables and animals, which are produced by solitary generation, gradually become more perfect, and at length produce a sexual progeny. (The Temple of Nature, Note on Reproduction, 1803)
Or they are changes produced by the mixture of species as in mules; or changes produced probably by the exuberance of nourishment supplied to the fetus, as in monstrous births with additional limbs; many of these enormities of shape are propagated, and continued as a variety at least, if not as a new species of animal. (Zoonomia Sect 39)
Here we see, in these brief selections from much longer books, Charles Darwin’s own grandfather talking about evolutionary theory some 60 years earlier. The phrases “Perhaps all the productions of nature are in their progress to greater perfection” and “gradually become more perfect” certainly have the kernel of the right idea for species creation. But it isn’t pursued to its logical end and therefore Erasmus didn’t really understand the essential power of vast numbers of one generation after another of progressive improvement, or he would have pursued it. Nor did he seem to grasp the idea that some separation of a unified species over a vast number of reproductions within a different environment would create new and non-interbreedable species from a formerly unified species. It wasn’t until the geologist James Hutton showed the vast depths of time that this idea would become apparent and thus speciation becomes reasonable.
See the Gutenberg Library for complete online texts of these books.
Charles Darwin owes more than his genes and money to Erasmus Darwin.