“Quant a moi, j’ai ete conduit, dans ma definition de l’espece, a mettre decidement la ressemblance au-dessus de caracteres de succession.  Ce n’est pas seulement a cause des circonstances propres au regne vegetal, dont je m’occupe exclusivement; ce n’est pas non plus afin de sortir ma definition des theories et de la rendre le plus possible utile aux naturalistes descripteurs et nomenclateurs, c’est aussi par un motif philosophique.  En toute chose il faut aller au fond des questions, quand on le peut.  Or, pourquoi la reproduction est-elle possible, habituelle, feconde indefiniment, entre des etres organises que nous dirons de la meme espece?  Parce qu’ils se ressemblent et uniquement a cause de cela.  Lorsque deux especes ne peuvent, ou, s’il s’agit d’animaux superieurs, ne peuvent et ne veulent se croiser, c’est qu’elles sont tres differentes.  Si l’on obtient des croisements, c’est que les individus sont analogues; si ces croisements donnent des produits feconds, c’est que les individus etaient plus analogues; si ces produits euxmemes sont feconds, c’est que la ressemblance etait plus grande; s’ils sont fecond habituellement et indefiniment, c’est que la ressemblance interieure et exterieure etait tres grande.  Ainsi le degre de ressemblance est le fond; la reproduction en est seulement la manifestation et la mesure, et il est logique de placer la cause au-dessus de l’effet.”

We are not yet convinced.  We still hold that genealogical connection, rather than mutual resemblance, is the fundamental thing—­first on the ground of fact, and then from the philosophy of the case.  Practically, no botanist can say what amount of dissimilarity is compatible with unity of species; in wild plants it is sometimes very great, in cultivated races often enormous.  De Candolle himself informs us that the different variations which the same oak-tree exhibits arc significant indications of a disposition to set up separate varieties, which becoming hereditary may constitute a race; he evidently looks upon the extreme forms, say of Quercus Robur, as having thus originated; and on this ground, inferred from transitional forms, and not from their mutual resemblance, he includes them in that species.  This will be more apparent should the discovery of transitions, which he leads us to expect, hereafter cause the four provisional species which attend Q. Robur to be merged in that species.  It may rightly be replied that this conclusion would be arrived at from the likeness step by step in the series of forms; but the cause of the likeness here is obvious.  And this brings in our “motif philosophique.”

Not to insist that the likeness is after all the variable, not the constant, element—­to learn which is the essential thing, resemblance among individuals or their genetic connection—­we have only to ask which can be the cause of the other.