I mention these two sciences together because it is by comparing them that their chief significance appears. It is one of the most astonishing discoveries of science and at the same time one of the most convincing proofs of evolution, that the whole process of the development of the human race from the lowest or simplest forms, which constitutes the subject-matter of phylogeny, is reproduced in brief in the development of the embryo of the individual. This remarkable fact Haeckel named “the biogenetic principle.” Darwin’s chief claim however to a pedestal in the hall of fame rests on his discovery of “natural selection.” During his memorable voyage on “The Beagle” he observed that there was no essential connection between a species’ reproductive powers and the number of its population. As this discovery plays an important part in his theory we will let him speak for himself. In his “Journal of Researches” he gives the following case, with his conclusion: “I was surprised to find, on counting the eggs of a large white Doris (a kind of sea slug) how extraordinarily numerous they were. From two to five eggs (each three thousandths of an inch in diameter) were contained in a spherical little case. These were arranged two deep in transverse rows forming a ribbon. The ribbon adhered to the rock in an oval sphere. One which I found, measured nearly twenty inches in length and half inch in breadth. By counting how many balls were contained in a tenth of an inch in the row, and how many rows in an equal length of the ribbon, on the most moderate computation there were six hundred thousand eggs. Yet this Doris was certainly not very common: although I was often searching under the stones I saw only seven individuals. No fallacy is more common among naturalists, than that the numbers of an individual species depend on its powers of propagation.” This instance is moderate compared with multitudes of others. The question then arises as to why, of such a numerous progeny, only a sufficient number reach adult stage as will replace the parent stock so that population remains practically stationary.