Page:Jardine Naturalist's Library Foreign Butterflies.djvu/42

40 'plastic virtue,' and other phantoms of the middle ages.

"It is evident, that if some well authenticated facts could have been adduced to establish one complete step in the process of transformation, such as the appearance in individuals descending from a common stock, of a sense or organ entirely new, and a complete disappearance of some other enjoyed by their progenitors, that time alone might then be supposed sufficient to bring about any amount of metamorphosis. The gratuitous assumption, therefore, of a point so vital to the theory of transmutation, was unpardonable on the part of its advocate ."

The transmutability of species is a point which has been maintained by many naturalists besides Lamarck, and the reasons they have adduced in support of their opinions are so various, that the full consideration of them would be inconsistent with our present purpose. It may be assumed as capable of most satisfactory proof, that the mutations which species undergo in accommodating themselves to a change of external circumstances, have a definite limit, and are regulated by constant laws; and that the capability of so varying, forms part of the specific character. Indefinite divergence from the original type is guarded against, in the case of intermixture of distinct species, by the sterility of the mule offspring; circumstances which show that