noids, the highest free forms begin with germs provided with a stem, 

 thus assigning, on embryological grounds, a lower position to all 

 those which are provided with a stem. 



In the same manner has it been possible to determine the position 

 of Bryozoa among Mollusca below Ascidioe, upon the ground that 

 their embryonic development is similar. It has been possible, in the 

 same way to assign to Pteropoda a position inferior to that of Gaste- 

 ropoda proper, and not intermediate between Gasteropoda and Cepha- 

 lopoda, as anatomical investigations would seem to indicate. For 

 it is now plain that the spreading appendages of the body of Ptero- 

 poda are not analogous to the long tentacles which encircle the head 

 in Cuttle-fishes, but correspond to the vibratory rudders of the em- 

 bryo in marine Gasteropoda. 



Again, the position of Foraminiferse, seems to me no longer doubt- 

 ful. They are neither microscopic Cephalophoda, nor Polypi, as of 

 late it has been generally, thought best to consider them, but con- 

 stitute a truly embryonic type in the great division of Gasteropoda, 

 exemplifying, in this natural division, in a permanent condition, the 

 embryonic state of development of common Gasteropoda, during 

 which the bulk of the yolk passes through the process of repeated 

 divisions. 



This principle of embryological changes as a foundation of the 

 natural classifications in the internal arrangement of all the minor 

 groups in the natural classes of the animal kingdom applies with 

 equal success to the Vertebrata. 



We need only contrast the successive changes of tailless Batra- 

 chians during their metamorphoses, with the permanent forms of the 

 caudate and branchiate types in that order, to be satisfied that the 

 relative rank of all these genera can in no way be better determined 

 than by a direct comparison of the permanent forms of the whole 

 group, with the successive changes in the embryonic condition of 

 its higher types ; and a comparison of the metamorphoses them- 

 selves, in the different genera, will leave no doubt as to which of 

 them the highest rank should be assigned. 



I have already, on other occasions, alluded to the improvements 

 which are likely to be introduced into our classification of birds, upon 

 considerations derived from embryological data. I may be permitted 

 here to add, that even the classification of mammalia will receive 

 decided improvements upon the consideration of their embryological 

 changes. A single instance, even now, will at least show that the 



