Rarity and endangerment: Why do they matter?

It is often supposed that valuable organisms are more valuable if they are rare. Likewise if they belong to endangered species. I consider what kinds of value rarity and endangerment can add in such cases. I argue that individual organisms of a valuable species typically have instrumental value as means to the end of preserving their species. This progenitive value, I suggest, tends to increase exponentially with rarity. Endlings, for their part, typically have little progenitive value; however, I argue that they may nonetheless have persistence value because, merely by existing, they postpone the numerical extinction of their species. Finally, I propose that a sentient endling can have higher lifeworld value than it would have had were it not the last of its kind. This, I argue, is because when a sentient endling dies, very little of its lifeworld is preserved – and this, I suggest, can be a bad thing.