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 gastraea, 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. 



