First of all, then, what is this distinction? Take, for example, the case of the Porto Santo rabbits. To almost every naturalist who reads what has been said touching these animals, it will have appeared that the connexion in which they are adduced is wholly irrelevant to the question in debate. For, it will be said that the very fact of the seemingly specific differentiation of these animals having proved to be illusory when some of them were restored to their ancestral conditions, is proof that their peculiar characters are not specific characters; but only what Mr. Wallace would term "individual characters," or variations that are not _inherited_. And the same remark applies to all the other cases which have been adduced to show the generality and extent of climatic variation, both in other animals and also in plants. Why, then, it will be asked, commit the absurdity of adducing such cases in the present discussion? Is it not self-evident that however general, or however considerable, such merely individual, or non-heritable, variations may be, they cannot possibly have ever had anything to do with the origin of _species_? Therefore, is it not simply preposterous to so much as mention them in relation to the question touching the utility of specific characters? Well, whether or not it is absurd and preposterous to consider climatic variations in connexion with the origin of species, will depend, and depend exclusively, on what it is that we are to understand by a species. Hitherto I have assumed, for the sake of argument, that we all know what is meant by a species. But the time has now come for showing that such is far from being the case. And as it would be clearly absurd and preposterous to conclude anything with regard to specific characters before agreeing upon what we mean by a character as specific, I will begin by giving all the logically possible definitions of a species. 1. _A group of individuals descended by way of natural generation from an originally and specially created type._ This definition may be taken as virtually obsolete. 2. _A group of individuals which, while fully fertile_ inter se, _are sterile with all other individuals--or, at any rate, do not generate fully fertile hybrids._ This purely physiological definition is not nowadays entertained by any naturalist. Even though the physiological distinction be allowed to count for something in otherwise doubtful cases, no systematist would constitute a species on such grounds alone. Therefore we need not concern ourselves with this definition, further than to observe that it is often taken as more or less supplementary to each of the following definitions. 3. _A group of individuals which, however many characters they share with other individuals, agree in presenting one or more characters of a peculiar kind, with some certain degree of distinctness._