Now there are three ideas in this law of natural selection: first, that there are inborn variations among the offspring even of the same family; secondly, that these various individuals living in surrounding conditions on the whole uniform and common to all of them, will start in life, some with an advantage and others with a relative disadvantage, and that those possessing an advantage will, more of them, tend to produce offspring; thirdly, that the variations, inborn in this case and not acquired, will probably be transmitted. That there are marked variations--physical, mental, and moral--among a litter of kittens or puppies is within the experience of everyone who has kept them, and that variations in human families are as marked is known to everyone who has brothers and sisters. Even twins frequently differ considerably from each other, and it is said that the last years of the lives of the Siamese twins were sadly marred by their opposing views as to the rights and wrongs of the American Civil War! It stands to reason also that these variations may be of advantage or disadvantage to their possessors, and that among animals and plants, where there are no social props given to the weak, the variations may and do determine survival. To give an idea of the rigorous operation of selection which we find among the lower animals, we have only to enumerate the number of the progeny produced by each pair, which is often prodigious, and knowing as we do that the number of individuals in a species remain virtually the same in a given district for long periods of time together, we conclude that the room of the parents is just filled by a younger pair, and all the excess of their progeny over and above this one pair must have succumbed to surrounding want and hardship. To give one concrete example out of hundreds that might be selected, let us take the case of the golden eagle given by Weismann in his essay on the “Duration of Life.” He says: “Let us fix the duration of life in the golden eagle at sixty years, and its period of immaturity (of which the length is not exactly known) at ten years, and let us assume that it lays two eggs a year, then a pair will produce one hundred eggs in fifty years, and of these only two will develop into adult birds, and thus on an average a pair of eagles will only succeed in bringing a pair of young to maturity once in fifty years; and so far from being an exaggeration, this calculation rather under-estimates the proportion of mortality among the young.”