It is not Darwin law.
It was found out from an experiment conducted by Harold and Urey. They assembled an atmosphere similar to that thought to exist on early days. (this had molecules like methane, ammonia and hydrogen sulphide but no oxygen over water) . This was maintained at a temperature less than 100 degree Celsius and sparks were passed through the mixture of gases to stimulate lightning. At the end of the week, 15% of carbon from ethane had been converted to simple compounds of carbon including amino acids which make up protein molecules. This way life arisen afresh on earth.