How i went about this:
even the much more central idea of species was little more than a theoretical
Bolded words tell me that the idea of species was very downplayed.
For the 1st blank we need to know whether race was an important conceptual category or not. Since even the more central idea of species was very downplayed and not important, we know race -- a less central idea -- can not be important. Hence, A works: Darwin did not require race...
Now, the noun in the second half of the sentence is IDEA. The idea of species is not a practice (a thing you do) and nor is it a face (it is a concept). Thus, E fits since ideas can be conveniences. We classify animals into species because it is convenient for the study of animals.
Thanks and if anyone here is knowledgeable and has free time, please look at the two questions I posted that have gone unanswered that i keep hoping someone will help with!