SYSTEMATIC DEVELOPMENT. 55 



family ; \vhence 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 



