I’m currently reading Darwin’s Voyage of the Beagle, and at one point he refers to a lizard as being an amphibian. We’d call lizards reptiles, so, since he was a rather knowledgable naturalist, I’m assuming there’s been a change in the terminology since the 1840s. The two groups used to be lumped together – when (and why?) did the split take place?
According to the Oxford English Dicionary, under “Amphibia”, Linnaeus grouped both together as “Amphibia”, but the first to split them in the modern way was Macleay in 1819. However, OED gives an 1859 cite which suggests that opinion was still divided at that time.
This is an outstanding question, and I’m very interested in seeing the answer.
I’ll note that in their original uses, the terms “reptile” and “amphibian” did not signify “member of the Class Reptilia/Amphibia” (respectively) but rather meant: 1. “(Backboned) animal which crawls upon the ground,” as in Genesis 1:25 – salamanders, like crocodiles and lizards, “creep” with splayed legs, as opposed to the upright posture of mammals and birds; and 2. “amphibious animal, equally at home on land or in the water.” Marine iguanas, crocodiles, and otters, like frogs, navigate equally well on land or sea, and would be amphibious (adj.) or amphibians (noun). It’s worth noting that even today the subdivision of zoology which studies reptiles and amphibians is herpetology, keeping them conjoined in one field of study. Essentially what you’re asking is when the restriction of the term to the individual classes came about.
Final note: Cladistics asserts that there is technically no such thing as a reptile. Clade Amniota includes all those creatures which reproduce by means of an amniote egg, whether laid on land or retained in the body (and sometimes nourished via a placenta). It includes all birds, all mammals, and a variety of living and extinct forms which do not have bird or mammal characteristics. This latter group, which does not constitute a valid clade, is what was formerly called Class Reptilia.
For those, like me, who aren’t up on cladistics, here’s a link that seems relatively untechnical.
It could have been as early (…or late?) as 1866, when Ernst Haeckel coined the term “Lissamphibia”, which today is the functional cladistic equivalent* to the old Class Amphibia (that is, contains what most folks consider “proper” amphibians such as frogs, salamanders and caecilians). Though the modern Lissamphibia excludes a lot of salamandery-looking critters, which are now classified as either basal temnospondyls or lepospondyls or limnarchs, which probably would have been considered amphibians by previous generations.
- Or should I say, it’s the closest we have to a proper amphibian clade. Temnospondyli might be closer, and would include all those aforementioned lepospondyls, limnarchs, and other basal critters, but few people have ever heard of those anyway. So for those critters which are known as amphibians in the vernacular, Lissamphibia includes all of them.
Just to elaborate a little, there *is * still a valid clade that is called Reptilia, but it does not correspond to the traditional “reptiles” (turtles, lizards, snakes, tuatara, and crocodilians). The present clade Reptilia includes the birds, and excludes various early amniotes previously classified as reptiles, included the synapsids (“mammal-like reptiles”), the ancestors of the mammals. Basically, it is the sister-group of the synapsid/mammal line.