SYSTEMATIC DEVELOPMENT. 55 
family ; whence arose the fixed idea that the embryo of 
the higher animals passes through the forms of the lower 
animals. When natural philosophy, more especially in 
Germany, had elaborated this doctrine in a rather fan- 
tastical manner, and had proclaimed that Man was the 
sum of all animals, in structure, as well as in develop- 
ment, “ the doctrine,” says Von Baer, “of the uniformity 
of individual metamorphosis with the vague metamor- 
phoses of the whole animal kingdom necessarily acquired 
great weight, when, by Rathke’s brilliant discovery, ger- 
minal fissures were demonstrated in the embryos of 
mammals and of birds, and the appropriate vessels 
were soon afterwards actually revealed,” 
The exaggerations and false inferences drawn from 
general analogies, and the vague ideas of types hover- 
ing above the whole, and regulating individual develop- 
ment, were wittily chastised by Von Baer. 
“To convince ourselves that a doubt as to this doctrine 
is not utterly groundless, let us imagine that the birds 
had studied the history of their development, and that 
it was they who now investigated the structure of the 
mature mammal and of man. Might not their physio- 
logical manuals teach as follows ?—‘ These quadrupeds 
and bipeds have much embryonic resemblance, for their 
cranial bones are separate; like ourselves during the 
first four or five days of hatching, they are without 
a beak; their extremities are tolerably like each other, 
as are ours for about the same time; not a single true 
feather is to be found on their bodies, only thin feather- 
shafts, so that, even in the nest, we are more advanced 
than they ever become; their bones are not very hard, 
and like ours, in our youth, contain no air at all; they 
