But where there are numerous species, new ones are forever being found. And things can get very tricky: sometimes experts have yet to figure out if animals are of the same or different species. Take mollusks.
但種類繁多、新種仍在不斷被記錄、甚至人類仍未為其找出最適合分類方式的物種，情況就變得較為棘手。