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 
