This is the idea that laws exist to guide behavior via threats of punishment and, when broken, authorize the use of state coercion and compulsion; there are social convention about who is authorized to make forward-oriented law and how; moreover, everything that the conventionally recognized authorities produce is law and nothing they do not produce is law.
Judges are then excused from lawmaking entirely. What, however, shall they do in hard cases?
In Elmer's case, there was no positive law that mandated that his inheritance be taken away from him. In this and other such cases, there is a simple conventionalist solution: always decide for the defendant. For whatever is not explicitly forbidden is permitted. If no law forbids Elmer to keep the loot, then Elmer is permitted to keep the loot. That's it.
Same with the McLoughlin case: if it is forbidden by an explicit law to cause emotional damage to a person in the specific circumstances of the case, then the wife is entitled to damages; if no such law is on the books, however, no damages should be awarded.
In the snail darter case, there were two laws that contradicted each other. In such cases, if there is a way to determine which law has precedence and ought to prevail, then it should be utilized. For example, a state law may nullify a federal law; or, on the contrary, a federal law may override a state law. A new statute always reliably overturns an old precedent. Etc., fine. If, however, no reasonable way of choosing between two contradictory laws exists, then a rational decision cannot be rendered, and the court should either refuse to decide at all, or again decide in favor of the defendant.
We might say that both laws cancel each other out and lose force. This is an unfortunate situation, as the people building the dam and the people seeking to protect the fish come to be at war with each other, with neither knowing whose will shall prevail. At that point, the appropriate legislature should be speedily convened and the contradiction resolved.