40 
MEMOIR OF LAMARCK. 
‘ 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 com - 
plete 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 en- 
joyed by their progenitors, that time alone might 
then be supposed sufficient to bring about any 
amount of metamorphosis. The gratuitous assump- 
tion, 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 muta- 
tions which species undergo in accommodating them- 
selves 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 
Frinciple8 of Geology, ii. p. 8. 
