French zoologist, proponent of the relatedness of species by internal structure.
Born in 1772 in Etampes, St. Hilaire grew up in an age in which science was much in vogue. The power of scientific observation to uncover non-evident truths about the world, demonstrated by tremendous technological advances, was becoming common wisdom among educated citizens. Scientific experiments became fashionable pastimes in the salons of the bourgeoisie.
One of the areas of great progress was the taxonomy of plants and animals. Scientists such as Carl von Linné, in attempting to classify all animals and plants into species, found ample proof that when you look at the similarities between species, they form a 'family' tree. For example, the chimpanzee and gorilla can be classified as apes; the apes can be classified as monkeys; the monkeys as mammals; the mammals as vertebrates; the vertebrates as animals. While this classification is obvious in most places, the details became a matter of serious scientific concern.
It was a sign of the times when the Jardin des Plantes ('Garden of Plants') in Paris was renamed to 'National Museum of Natural History', and a recognition of the importance of the work of St. Hilaire, who held a position there as professor of vertebrate zoology.
From 1794, he worked with naturalist Georges Cuvier on classifying the animals and plants Cuvier brought from his expeditions.
In 1809, St. Hilaire became professor of zoology at the University of Paris, a position he held until his death in 1844.
St. Hilaire was mostly concerned with the study of homology, the correspondence in internal structure of body parts between different species. For example, a human hand and a bat's wing have the same bone structure, even though they have different functions. St. Hilaire found such cases of homology everywhere, and tried to establish exact criteria to determine structural similarity.
This research was driven by a developing scientific debate with Cuvier and others on whether function follows form or form follows function in biology.
Cuvier stressed the second position: the body parts of a species are shaped and structured to fit their functions within the whole. He was not overly impressed with St. Hilaire's quest for homology, and rightly dismissed some of his proposed cases as unjustifiable from the evidence.
St. Hilaire emphasized the importance of homology,
and actually went as far as stating that all animals essentially share the same internal structure, modified to suit their function for the species.
Eventually, this made him ponder the logical conclusion, namely, that structural similarities between species can be explained by assuming that the species within the same family actually descend from a common ancestor, and develop their differences through a process of gradual diversification and adaptation.
Clearly, a heretical view, and one that Cuvier did not share.
It wasn't until 15 years after St. Hilaire's death that this view was put forward as a serious and well-researched scientific hypothesis, by Charles Darwin, in On the origin of species by means of natural selection (1859).
(As usual, the content, but not the wording, of this writeup was shamelessly stolen from the first hit on Google;
in this case,
Read it if you want more details.)