102 Darwin, and after Darwin. 
similar processes of development in the life-histories 
of their respective species—in so far, I mean, as the 
two sets of changes admit of being proved parallel. 
In the only illustration hitherto adduced—viz. that 
of deers’ horns—the series of changes from a one- 
pronged horn to a fully developed arborescent antler, 
is a series which takes place during the adult life of 
the animal; for it is only when the breeding age 
has been attained that horns are required to appear. 
But seeing that every animal passes through most of ° 
the phases of its development, not only before the 
breeding age has been attained, but even before the 
time of its own birth, clearly the largest field for 
the study of individual development is furnished by 
embryology. For instance, there is a salamander 
which differs from most other salamanders in being 
exclusively terrestrial in its habits. Now, the young 
of this salamander before their birth are found to 
be furnished with gills, which, however, they are never 
destined to use. Yet these gills are so perfectly 
formed, that if the young salamanders be removed 
from the body of their mother shorily before birth, 
and be then immediately placed in water, the 
little animals show themselves quite capable of 
aquatic respiration, and will merrily swim about in 
a medium which would quickly drown their own 
‘parent. Here, then, we have both morphological and 
physiological evidence pointing to the possession of 
gills by the ancestors of the land- salamander. 
It would be easy to devote the whole of the present 
chapter to an enumeration of special instances of the 
kinds thus chosen for purposes of illustration; but 
as it is desirable to take a deeper, and therefore 
