Research generally gets more difficult over time, in a quantifiable way, as you clear out the low-hanging fruit.

Hard to find

The fact that discovery can become extremely hard does not mean that it stops, of course. All three of these fields have continued to be steadily productive. But it does tell us what kind of resources we may need to continue discovering things. To counter an exponential decay and maintain discovery at the current pace, you need to meet it with a scientific effort that obeys an exponential increase. To find a slightly smaller mammal, or a slightly heavier chemical element, you can’t just expend a bit more effort. Sometimes you have to expend orders of magnitude more.