Darwin's Hypothesis of Evolution by Natural Selection included the observation that populations often grow faster than the food supply and therefore individuals need to compete for resources. Darwin borrowed this idea from the work of:

Thomas Malthus on  his essay on population.

Explanation:

Malthus concluded that human population grow faster than the available resources in their environment.However,no matter how this population increases,selection pressure like famine,natural disasters,always  keep the population in check,thus  maintaining the population size within moderate levels.

He believed the weakest in the population (with little resistance to the selection pressure) will die,thereby reducing the population, encouraging the survival of the fittest,and stabilizing and preventing expulsion.

Darwin, assumed that the surviving  organisms in the population must have inherited certain characteristics/traits  from their parents,which make them to resist the selection pressures.He called the surviving ones the survival of the fittest by natural selection.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.