ON. THE DISTINCTION OF SPECIES. 619 
suspect their identity would be proved by a sufficiently care- 
ful examination of the former in its native localities. "This 
opinion is much strengthened by the frequent appearance of 
characters assigned to E. Mackaii among plants of E. Tetra- 
liv : the ovate leaves, their glabrous upper surface and (more ` 
rarely) mid-rib beneath, and their crowded position on the 
flowering shoots, may all be found on plants of E. Tetraliz. 
Fourthly, characters which are common to two book spe- 
cies in the earlier stages of their development, but which are 
lost by one of them in its progress to maturity, are not suf- 
ficient proofs of their distinctness as natural species. "The 
uncertainty of such distinctive characters is occasionally de- 
monstrated by their permanence or re-appearance in indivi- 
dual plants of the one book species which usually loses them ; 
or, vice versá, by their disappearance in individuals of the 
other book species, which commonly retains them. For ex- 
ample, Veronica hirsuta (Hopkirk) is distinguished from 
V. officinalis, chiefly by its entire capsules, as opposed to the 
emarginate capsules of the latter. In both book species the 
capsules are entire at first, those of V. officinalis becoming 
emarginate in their progress to maturity. But I have gather- 
ed strong plants of the latter, in which the full-grown cap- 
~ sules retained the infant-form, being obovate and not in the 
least degree emarginate. And I possess also a specimen of 
V. hirsuta, from the Botanic Garden of Edinburgh, on which 
there is a small capsule, slightly emarginate. I regard them 
as constituting only one species in nature. Even Mr. Babing- 
ton unites them, although it is done under the saving clause, 
applied to V. hirsuta, * probably a distinct species, since it 
retains its characters in cultivation.” The varieties of Betula 
alba afford another example. Betula glutinosa (Fries) is said, 
in the Manual of British Botany, to be readily distinguished 
from B. alba, by its * cordate-ovate" leaves, those of the 
letter being * rhomboid-triangular.” Betula pubescens 
(Ehrh.) is there described as a variety of B. glutinosa, known 
by its downy dec peduncles and young twigs. Now, the 
