How do things get their names? Ricin is a by-product of the production of castor oil, right? and the Latin term for "castor oil" is ricinus communus. But whence castor oil? seeing as "castor" is part of the Latin name of the humble beaver (Castor canadensis), which has nothing whatever to do with castor oil...or with ricin....How do people determine how a thing gets its name from a Latin derivation?

I mean, like, why ricin? and why castor oil, which derives from the same source and therefore one might think the name would derive from the Latin, too?

nothing whatever to do with castor oil

AHD speculates that castor oil may have been so named from the substitution of its oil for that derived from the beaver for medicinal purposes.

so, then, what was the beaver oil called? (if I'm reading your post kerrecktly, and they used to use the beaver oil, before they substituted the other and started calling it - the other, that is - castor oil?) was it also castor oil? one of those cases where the thing retains the name despite no longer containing the ingrediments suggested by the name...?

Apparently the stuff they got from the beaver was called castor. YCLIU.

So castor oil is just another incidence (or perhaps the earliest? or one of the earliest?) of false advertising!

One site I saw called the oil from beavers castoreum.I'll go look again.
Found it:
http://www.conservation.state.mo.us/conmag/1999/01/1.html

Hel-lo, look what Atomica says:
cas·tor1 (kăs'tər)
n.
1. An oily, brown, odorous substance obtained from glands in the groin of the beaver and used as a perfume fixative.
2. A hat made of beaver fur or an imitation.
3. A heavy wool fabric used especially for overcoats.
[Middle English, from Latin, beaver, from Greek kastōr.] I L'dIU because Dr. Bill's article refers to the "castor gland", and I thought that's what it might be called. Mercy me--after seeing #1, I may never wear perfume again; and I have never heard of hats or wool being referred to as castors.

3. A heavy wool fabric used especially for overcoats.

I've come across that usage lately and didn't know what it meant. Now I do. Thanks Jackie!