TYPES OF DEVELOPMENT, 49 
and in a certain sense, by a particular gradual evolution 
of the organs, a bird also, but made the embryo like- 
wise repeat and surpass the lower types. To this false 
tendency, acting on vague analogies, a stop was put by 
the great naturalist just named. He showed that a 
number of coincidences might, indeed, be demonstrated 
between the embryo of the higher and the permanent 
form of the lower animals, but that this resemblance 
rested essentially on the fact that in the embryo of the 
higher animal the differentiation of the general funda- 
mental mass had not yet set in, and that in the progress 
of development it passes through stages which are per- 
manent in the series of inferior animals. 
On the other hand, he positively repudiated the asser- 
tion that the embryos of the higher types actually pass 
through forms permanent in the lower ones. He says 
that the type of each animal seems from the first to 
fix itself in the embryo, and to regulate its whole 
development. As regards the vertebrate animals in 
particular, the further we go back in the history of their 
development, the more do we find the embryos alike, 
both on the whole and in the individual parts. “Only 
gradually do the characters appear which mark the 
greater, and later those which mark the smaller divi- 
sions of the Vertebrata. Thus from the general type 
the special one is evolved.” 
Von Baer thus held that the analogy consisted only in 
the embryonic states of the various animal forms; but he 
was obliged to go beyond the circle of the types, and he 
thought it. probable that.among all embryos of verte- 
brate, as well as invertebrate animals, developed from a 
true ovum, there is a conformity in the condition of the- 
