He found that creatures did indeed adapt though he didn't know how because, in his time, people didn't know about genes. It turns out that genes can change. They don't always make perfect copies when a cell divides. Sometimes a mistake is made. This is called a mutation.
Most mistakes are disasters - the organism dies.
Some are useful and give the creature an advantage which allows it to have babies which are more likely to be successful. But this process is entirely accidental. This slow business of trying new body shapes and styles, some of which work and some of which don't, has been going on for as long as life. Perhaps you know about it. It's called evolution - the survival of the fittest.