158 E VOL UTION OF TO-DA Y. 



has arisen, and this gastrula represents for our em- 

 bryologists the first multicellular animal which ever 

 existed, and which was, therefore, the common an- 

 cestor of all animals now living, with perhaps the 

 exception of the sponges. 



If it be asked whether this hypothetical ancestor 

 is represented by fossils, the answer is that it is not 

 and doubtless never will be ; its body, having no 

 hard parts, could not possibly have been preserved, 

 If it be asked whether it is a purely hypothetical 

 form, unlike any thing existing to-day, the answer 

 is, while the gastrula as such does not exist to-day as 

 an adult animal, there are some ccelenterates which 

 approach quite close to it and are to be considered 

 as direct modifications of this ancestor. 



The Separation of the Sub-Kingdoms. 



Thus far we have been able to trace the embry- 

 ology of all animals over the same road, but we can 

 do this no farther. From the gastrula the different 

 large groups depart in different directions. This 

 point has only of late years been recognized. The 

 understanding of the animal kingdom at the first of 

 the century was such that all of the different groups 

 were placed in linear order one above the other. 

 If this were true the embryology of the higher ani- 

 mals ought to be much the same as that of the 

 lower ones, as far as the adult condition of the 

 latter. But this idea proved palpably untenable, and 

 soon was replaced by the conception of the animal 

 kingdom under the form of a tree, whose branches, 

 as we go downward, continually unite into large 



