The only other line of argument, which has any show of probability, is that based upon _the gradations of the various orders of plants and animals_.

Not but that there are many other arguments adduced, but they are of too technical a character to be intelligible to any but zoologists, and of too little weight to demand consideration after the leading arguments are overturned.

But this argument from gradation, though logically unsound, is plausibly specious, and therefore demands notice.
By far the ablest exhibition of this argument is that made by Lamarck, and we give it as he presents it: "The greater the abundance of natural objects assembled together, the more do we discover proofs that everything passes by insensible shades into something else; that even the more remarkable differences are evanescent, and that nature has for the most part left us nothing at our disposal for establishing distinctions, save trifling, and in some respects puerile particularities.

We find that many genera among plants and animals are of such an extent, in consequence of the number of species referred to them, that the study and determination of these last have become almost impracticable.

When the species are arranged in a series, and placed near to each other, with a due regard to their natural affinities, they each differ in so minute a degree from those next adjoining, that they almost melt into each other, and are in a manner confounded together.