My dear Darwin/

You wont let me off without definitions, you say!. I will try you with something between a definition and an explanation of what I mean by a typical species—*¹ The term “typical species” is used by Zoologists in two senses—it either refers to that species which possesses in the highest degree of developement some of the characters which distinguish the group xx² to which it belongs from other groups; or, it has reference to that species which is supposed to exhibit, in the best balanced condition, the greatest number of characters most common to the species forming the group of which it is a member— In the former case the type of the group would be that species which is most removed from other groups but in the latter such would not be the case—

By way of illustration I will suppose I am called upon to point out a type of the order Carnivora. According to the first definition I should select a Cat because in the Cat tribe some of the more striking characters of the Carnivora are most strongly developed; but, were I to adopt the second definition I should choose a Viverra because it may be said to possess most evenly developed the greatest number of characters which are found in the species of its order; and, in a Natural classification, such differences should (according to my views) be expressed by placing the Cats, among the Carnivora at the confines of the order which are most removed from the groups forming the orders which are most nearly related to the Carnivora, whilst the Viverra should be placed in the middle of the carnivorous order— I would distinguish the two, so called, types by terming the one a type of a carnivorous Mammal, and the other a type of the Order Carnivora— I will not however take upon myself to say that a type is a thing which exists in Nature—it may only be an abstract idea— It may be impossible to name any particular species which would be generally admitted as a type according to either of the two views above referred to, but it would not be difficult to show that some particular species approaches very near to the idea of the type in either case—

It may help to make my notions clear if I remind you of the general law of developement of parts in animals, viz. that when one organ is greatly developed it is at the expense, as it were, of some other organs—³ thus the carnassial tooth, so characteristic of the order Carnivora, being a much developed tooth, other neighbouring teeth are robbed of their share of nutriment by it, and in the Cat, which has the carnassier most developed, the false and true molar teeth are least developed— It has in fact no true molar teeth in the lower jaw, and but a rudimentary one on either side of the upper jaw, but taking the order Carnivora as a whole the species could not be characterized as being almost destitute of true molar teeth for the greater portion of the species have two tolerably well developed true molars on each side of the upper jaw, and all, with the exception of the Cats have one at least on each side of the lower jaw— From this it will be perceived that the Cats furnish an exception among the Carnivora to the most common characters of the dentition, and so far would not would not serve as a good illustration of the order, though they might furnish the best illustration of a flesh-eating Mammal— On the other hand the Viverridæ have two true molars (let it always be understood I mean on both sides of the jaws) in the upper jaw and one in the lower (combined with a tolerably well developed carnassier) in which character they agree with the Ursidæ (but here there is no tooth which can functionally be called a carnassier) and are intermediate between the Canidæ and the Mustelidæ, the former of these two families having two molars to the upper jaw and ditto to the lower whilst the Mustelidæ have one to each jaw— So much for the dentition and were I to speak of other parts of structure in the Viverras I could show that they are intermediate between the Dogs, Bears, Cats and Weasels— I should therefore call them the typical family of the order Carnivora, and that species of the Viverridæ which was most removed from the Dogs, Bears, Cats and Weasels, the type of the family Viverridæ—*⁴

Believe me | Ever Yours | Geo. R. Waterhouse *Species are the supposed descendants from a common parent. Animals are said by Zoologists to be of the same species when they perfectly resemble each other, or when they differ only in degree so far as from experience they have been found to differ in animals of the same parents—those parents being alike—

^xxBy “group” I mean any assemblage of species—

* In a classification the family Viverridæ would be central,⁵ the families of Dogs, Bears Cats & Weasels being arranged around it—and the type of the family Viverridæ would also be central being the most removed from the species of the other families, and according to the same system the type of a Carnivorous animal would, as I have before stated, be external. I know the question you are going to put to me now you have read this last paragraph! writing it has opened my eyes to a point for consideration from which something may spring.

Qy— What would be the type of a Central group, like the Viverridæ, according to my first definition? I dont know one—xx—but I am not answerable for that, for I have never used the word type but in accordance with the second definition— this notion has just come into my head but I cant think it out for I am very sleepy—past two o’clock!!