Embryology. 103 
a more general view of the whole subject, I will begin 
at the foundation, and gradually work up from the 
earliest stages of development to the latest. Before 
starting, however, I ask the reader to bear in mind 
one consideration, which must reasonably prevent 
our anticipating that in every case the life-history of 
an individual organism should present a fud/ recapitu- 
lation of the life-history of its ancestral line of species. 
Supposing the theory of evolution to be true, it must 
follow that in many cases it would have been more or 
less disadvantageous to a developing type that it 
should have been obliged to reproduce in its individual 
representatives all the phases of development pre- 
viously undergone by its ancestry—even within the 
limits of the same family. We can easily understand, 
for example, that the waste of material required for 
building up the useless gills of the embryonic sala- 
manders is a waste which, sooner or later, is likely to 
be done away with; so that the fact of its occurring 
at all is in itself enough to show that the change from 
aquatic to terrestrial habits on the part of this species 
must have been one of comparatively recent occurrence. 
Now, in as far as it is detrimental to a developing 
type that it should pass through any particular ances- 
tral phases of development, we may be sure that natural 
selection—or whatever other adjustive causes we may 
suppose to have been at work in the adaptation of 
organisms to their surroundings—will constantly seek 
to get rid of this necessity, with the result, when 
successful, of dropping out the detrimental phases. 
Thus the foreshortening of developmental history 
which takes place in the individual lifetime may be 
expected often to take place, not only in the way of 
