Embryology. 155 
the eight animals at the earliest of the three stages 
represented—all having fish-like tails, gill-slits, and 
so on. In the next stage further differentiation has 
taken place, but it will be observed that the limbs 
are still so rudimentary that even in the case of Man 
they are considerably shorter than the tail. But in 
the third stage the distinctive characters are well 
marked. 
So much then for an outline sketch of the main 
features in the embryonic history of the Vertebrata. 
But it must be remembered that the science of com- 
parative embryology extends to each of the other three 
great branches of the tree of life, where these take 
their origin, through the worms, from the still lower, | 
or gastreea, forms. And in each of these three great 
branches—namely, the Echinodermata, the Mollusca, | 
and the Arthropoda—we have a repetition of just the — 
same kind of evidence in favour of continuous descent, 
with adaptive modification in sundry lines, as that 
which I have thus briefly sketched in the case of the 
Vertebrata. The roads are different, but the method 
of travelling is the same. Moreover, when the em- 
bryology of the Worms is closely studied, the origin 
of these different roads admits of being clearly traced. 
So that when all this mass of evidence is taken to- 
gether, we cannot wonder that evolutionists should 
now regard the science of comparative embryology as 
the principal witness to their theory. 
