An objection will at once be raised to this illustration, to the effect that all zoologists admit the possibility of new Varieties, or Races, being formed; but they deny that new Species can be formed. It is here that the equivoque of the word Species prevents a clear understanding of each other’s argument. Whiteness may justly be said to be unalterable; but white things may vary—they may become gray, or yellow. In like manner Species must be invariable, because Species is a word indicating a particular group of characters; but animals may vary in these characters: they may present some of the characters less, or more, developed; and they may even want some of them. Now as there is no absolute standard of what constitutes Species, what Sub-species, and what Varieties, it becomes impossible to say whether any individual variation in an animal form shall constitute a new Variety, or a new Species. With regard to dogs the differences between the various races are so numerous, and so marked, as would suffice to constitute species and even genera, in other groups of animals. We must relinquish the idea of proving anything by the paintings and sculptures of the ancients. When we find an Egyptian plough closely resembling the plough still in use in some places, we may identify it as of the same “Species” as our own; but this does not disprove the fact that steam-ploughs, and ploughs of various construction, have been since invented, all of them being modifications of the original type. Formerly, and for many years, the stage-coach was our approved mode of conveyance—and it is still kept up in some districts; nevertheless, modifications of coach-road into tramroad, and tramroad into railroad, have gradually resulted in a mode of conveyance utterly unlike the stage-coach. It is the same with animals.