The following definitions of species have been selected by Dr. Bachman, and may be received as unexceptionable as any others; but we shall show that they fall far short of the true difficulties of the case. "We are under the necessity of admitting the existence of certain forms, which have perpetuated themselves, from the beginning of the world, without exceeding the limits prescribed: all the individuals belonging to one of these forms constitute a _species_."--CUVIER. "We unite under the designation species all those individuals who mutually bear to each other so close a resemblance as to allow of our supposing that they may have proceeded originally from a single being, or a single pair."--DE CANDOLLE. "The name species is applied to an assemblage of individuals which bear a strong resemblance to each other, and which are perpetuated with the same essential qualities. Thus man, the dog, the horse, constitute to the zoologist so many distinct species."--MILNE EDWARDS and ACHILLE COMPTE. We have no objection to this definition, but the examples cited are points in dispute, and not received by many of the leading naturalists of the day. "Species are fixed and permanent forms of being, exhibiting indeed certain modes of variation, of which they may be more or less susceptible, but maintaining throughout those modifications a sameness of structural essentials, transmitted from generation to generation, and never lost by the influence of causes which otherwise produce obvious effects. _Varieties_ are either accidental or the result of the care and culture of man."[198]--MARTIN. Dr. Bachman gives another, substantially the same, from Agassiz; and also one of his own, to which he appends, as an additional test of species, the production of "_fertile offspring by association_." In this definition the doctor _assumes_ one of the main points in dispute. "_Varieties_," says Dr. Bachman, "are those that are produced within the limits of particular species, and have not existed from its origin. They sometimes originate in wild species, especially those that have a wide geographical range, and are thus exposed to change of climate and temperature," &c. * * * "_Permanent varieties_ are such as, having once taken place, are propagated in perpetuity, and do not change their characteristics unless they breed with other varieties." We may remark that the existence of such _permanent varieties_ as here described is also in dispute. The same author continues:--