Vit “THE-ORIGIN OF SPECIES ” 237 
dency of biological investigation, since 1859, has 
been in the direction of removing the difficulties 
which the apparent breaks in the series created 
at that time; and the recognition of gradation 
is the first step towards the acceptance of evolu- 
tion. 
As another great factor in bringing about the 
change of opimion which has taken place among 
naturalists, I count the astonishing progress which 
has been made in the study of embryology. 
Twenty years ago, not only were we devoid of any 
accurate. knowledge of the mode of development 
of many groups of animals and plants, but the 
methods of investigation were rude and imperfect. 
At the present time, there is no important group 
of organic beings the development of which has 
not been carefully studied; and the modern 
methods of hardening and section-making enable 
the embryologist to determine the nature of the 
‘process, in each case, with a degree of minuteness 
and accuracy which is truly astonishing to those 
Whose memories carry them back to the 
beginnings of modern histology. And the results 
of these embryological investigations are in com- 
plete harmony with the requirements of the 
doctrine of evolution. The first beginnings of all 
the higher forms of animal life are similar, and 
however diverse their adult conditions, they start 
from a common foundation. Moreover, the pro- 
cess of development of the animal or the plant 
