712 LIFE : OUTLINES OF GENERAL BIOLOGY 



and no others, and which keep themselves to themselves, not 

 readily breeding with other species! Thus the raven, the carrion 

 crow, the rook, and the jackdaw are all species of the genus Corvus. 

 If a particular species, such as a weasel {Mustela nivalis) is satis- 

 factorily defined, and is really worthy of having a second name 

 {nivalis) all to itself, then (i) it should show a certain constancy in 

 its distinctive features, breeding true from generation to generation; 

 (2) its distinguishing features should be greater than those which 

 occur in a family circle, i.e. the offspring of a pair; (3) its members 

 should always be fertile with one another, and not readilj' with 

 those of a related species ; and (4) the species-characteristics should 

 be demonstrably intrinsic and not due to the modifying influences 

 that similar surroundings, food, or habits exert on successive crops 

 of individuals. These, as we have just explained, are the four criteria 

 of what may be called a "good species". 



But within a species it is often possible to distinguish minor 

 groups of similar individuals also breeding true, and these are called 

 sub-species or races or varieties; and it is often convenient to mark 

 them by a third name tacked on to the second name of the species. 

 Thus we call the Mountain Hares of the Scottish Highlands by the 

 name Lepiis timidus .scoticus and the Mountain Hares of the Alps, 

 L. timidiis varronis. That is to say, they are two sub-species of 

 the Scandinavian species L. timidus; and though some naturalists 

 laugh at these three-barrelled names, there is much to be said for 

 them. Thus, in the case noted, they indicate right away that the 

 Mountain Hares or Blue Hares of the Cairngorms on the one hand 

 and of the Alps on the other, are much nearer to one another than 

 they are to the Brown Hare, L. europcBus, or to the Irish Hare, 

 L. hihernicHs. This kind of question is not verbal at all; it is a 

 matter of clear thinking and clear seeing ; and it has been admirably 

 discussed afresh by Mr. G. C. Robson in his book, Tlie Species 

 Problem — a fine example of clear-headed scientific scholarship and 

 patient inquiry. But what we wish to discuss now is not the defini- 

 tion of a species, but how species arise — and we must not forget that 

 they are arising even in our midst to-day. Here we are, dehberately, 

 in part anticipating the evolutionary discussion of a later chapter. 



An understanding and acceptance of the evolution idea must have 

 been hindered by the frequent use of the French word trans- 

 formisme. which suggests the erroneous idea that one species turns 

 into another. To this day we hear ill-informed anti-evolutionists 

 demanding to be shown a case of one species turning into another. 

 But that is not what happens; it would be magic! What does 

 happen is that novelties or variations arise within a species, diverging 

 on a path of their own — a path which may lead to destruction, or, 

 in other cases, to the establishment of a new sub-species or variety 

 or race, and by and by, perhaps, a new species, that is to say, a 



