Research generally gets more difficult over time, in a quantifiable way, as you clear out the low-hanging fruit.
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.
Discovery is not difficult. Discovery is insubordinate. Discovery exists in no business plan, no PERT chart, no DCF/ROI. Discovery is (risk)^2 and therefore… rather than foster brilliance we allocate for its suppression. B-School makes it so. Example:
Young pisser untenured faculty at Moo U is researching the effects of electric fields on dividing cells, a perfect PERT chart project. Exercise process, churn out paper or three on schedule, grant a degree, be forgotten. Being basically, ah, innocent of chemistry he uses active platinum electrodes immersed in the culture medium instead of passive Parylene-C insulated capacitor plates with zero current flow. Unknown to him, Pt is not electrochemically inert in the presence of Lewis base coordinating species like amines and choride – your typical culture medium dosed with metabolic muck.
What should have been a dry labable result sprung forth with all sorts of bizarre cell forms and then high mortalities. Being ignorant but not stupid, he insubordinately traced down the source rather than published the naked results. The wholly unacceptable bottom line was cisplatin, square-planar cis-diaminodichoroplatinum(II), for treating cancer.
It turns out pretty much any geminal pair of labile groups on a (large radius) metal center will crosslink DNA during replication. Even titanocene dichloride works. Platinum in particular is death to kidneys. Moo U ate him up for patent rights and their income. His discovery was thus trivialized, fraught with human suffering, and others went water skiiing as he rowed the boat. One imagines they tossed him tenure with a stern warning to eat his own brain and never discover anything ever again.