Darwins explanation for evolution was dependent on which specific concept

The development of_ factored in the development of complex organisms.

Are these strong bases, neutral, strong acids, weak acids or weak base household bleach (ph=12.6) fluid in a car's battery (ph of less than 1.0) milk (ph=6.7) stomach contents (ph=2.2) pure water (ph=7)

Which layer will become saturated first when rain falls?