After concluding that the distinguishing attributes that bifurcate "non-human animals" and "animals" (humans) are ability to reason and capacity to suffer, Dawkins and Singer determine that there is more moral reprehensibility in terminating the life of a horse than a human fetus. Singer goes on to suggests that eating meat without thought to how the livestock is reared and slaughtered is akin to Germans turning their heads as Jews were slaughtered in droves. Additionally, the gravitas for deciding whether one should eat meat should be to the same degree one decides to have an abortion. At which point, Dawkins concedes the moral high ground to Singer—as Dawkins confesses his taste for meat. As to whether a line should be drawn on grounds for ending life, both are agreed that an exact line cannot be set, and propose a continuum in its stead. Behold the Darwinian perspective—without regard to the less than sanguine thesis of Darwinian evolution, i.e., survival of the fittest—sans image of God, dominion over the earth, and an impetus for love of neighbor.
I will close with the words of the inimitable Chesterton:
Nobody has any business to use the word 'progress' unless he has a definite creed and a cast-iron code of morals... For progress by its very name indicates a direction; and the moment we are in the least doubtful about the direction, we become in the same degree doubtful about the progress. Never perhaps since the beginning of the world has there been an age that had less right to use the word “progress” than we. - G.K. Chesterton, Heretics, 16.