90 GRADATION AMONG ANIMALS. 



tomical features in the adult that we must decide 

 this question. We must examine it also from 

 the embryological point of view. Every animal 

 in its growth undergoes a succession of changes : 

 is there anything in these changes implying a 

 transition of one type into another ? Baer has 

 given us the answer to this question. He has 

 shown that there are four distinct modes of de- 

 velopment, as well as four plans of structure ; 

 and though we have seen that higher animals of 

 one class pass through phases of growth in which 

 they transiently resemble lower animals of the 

 same class, yet each one of these four modes of 

 development is confined within the limits of the 

 type, and a Vertebrate never resembles, at any 

 stage of its growth, anything but a Vertebrate, 

 or an Articulate anything but an Articulate, or a 

 Mollusk anything but a Mollusk, or a Radiate 

 anything but a Radiate. 



Yet, although there is no embryological transi- 

 tion of one type into another, the gradations of 

 growth within the limits of the same type and the 

 same class, already alluded to, are very striking 

 throughout the Animal Kingdom. There are 

 periods in the development of the germ in the 

 higher members of all the types, when they 

 transiently resemble in their general outline the 

 lower representatives of the same type, just as we 

 have seen that the higher orders of one class 



