The reader is now in a condition to appreciate the general line of argument adopted in the discussion of Mr. Darwin’s book, which is at present exciting very great attention, and which will, at any rate, aid in general culture by opening to many minds new tracts of thought. The benefit in this direction is, however, considerably lessened by the extreme vagueness which is commonly attached to the word “species,” as well as by the great want of philosophic culture which impoverishes the majority of our naturalists. I have heard, or read, few arguments on this subject which have not impressed me with the sense that the disputants really attached no distinct ideas to many of the phrases they were uttering. Yet it is obvious that we must first settle what are the facts grouped together and indicated by the word “species,” before we can carry on any discussion as to the origin of species. To be battling about the fixity or variability of species, without having rigorously settled _what_ species is, can lead to no edifying result. It is notorious that if you ask even a zoologist, _What_ is a species? you will almost always find that he has only a very vague answer to give; and if his answer be precise, it will be the precision of error, and will vanish into contradictions directly it is examined. The consequence of this is, that even the ablest zoologists are constantly at variance as to specific characters, and often cannot agree whether an animal shall be considered of a new species, or only a variety. There could be no such disagreements if specific characters were definite: if we knew _what_ species meant, once and for all. Ask a chemist, What is a salt? What an acid? and his reply will be definite, and uniformly the same: what he says, all chemists will repeat. Not so the zoologist. Sometimes he will class two animals as of different species, when they only differ in colour, in size, or in the numbers of tentacles, &c.; at other times he will class animals as belonging to the same species, although they differ in size, colour, shape, instincts, habits, &c. The dog, for example, is said to be one species with many varieties, or races. But contrast the pug-dog with the greyhound, the spaniel with the mastiff, the bulldog with the Newfoundland, the setter with the terrier, the sheepdog with the pointer: note the striking differences in their structure and their instincts: and you will find that they differ as widely as some genera, and as most species. If these varieties inhabited different countries—if the pug were peculiar to Australia, and the mastiff to Spain—there is not a naturalist but would class them as of different species. The same remark applies to pigeons and ducks, oxen and sheep.