Should "either" be placed before or after the preposition in the following kind of sentences?
That species of fish live either in rivers or (in?) lakes. / That species of fish live in either rivers or lakes.
The diet should contain protein either from animal or (from?) plant sources. / The diet should contain protein from either animal or plant sources.
Thanks)

Question

Should "either" be placed before or after the preposition  in the following kind of sentences?
That species of fish live either in rivers or (in?) lakes. / That species of fish live in either rivers or lakes.
The diet should contain protein either from animal or (from?) plant sources. / The diet should contain protein from either animal or plant sources.
Thanks)

Answer

That species of fish lives in rivers and lakes. 
That species of fish lives in rivers and lakes.
There is no 'either/or' relationship here

The diet CAN contain protein either from animal or from plant sources.
The diet CAN contain protein from either animal or plant sources.

Answer

Either....or is used to refer to a choice of A or B, but not both. 'You can have either chicken or pork for dinner.' So as Eliot said your examples are probably not relevant to the use of 'either...or'. Having said that if the same preposition is used for A and B, it is not necessary to repeat it. 'Put the meat either in the fridge or (in) the freezer'

Answer

Here are the best ones:

Their diet contains protein from either animal or plant sources.

That species of fish lives in BOTH rivers or lakes.  If you say either it means you are not sure where it lives. If you say both it means, some live in rivers and some live in lakes or all of them live in rivers and lakes ;)

J

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