60 Evolution and Adaptation 
region. Meckel compares the bones of the higher verte- 
brates with the simpler bones of the lower forms, and even 
with the cartilages of the cephalopod. He points out that in 
the early human embryo the nerve cord extends the whole 
length of the spinal canal. He compares the simple heart of 
the embryo with that of worms, and a later stage, when two 
chambers are present, with that of the. gasteropod mollusk. 
The circulation of the blood in the placenta recalls, he says, 
the circulation in the skin of the lower animals. The lobu- 
lated form of the kidney in the human embryo is compared 
with the adult condition in the fishes and amphibians. The 
internal position of the reproductive organs in the higher 
mammals recalls the permanent position of these organs in 
the lower animals. The posterior end of the body of the 
human embryo extends backwards as a tail which later dis- 
appears. 
Some of these comparisons of Meckel sound very absurd 
to us nowadays, especially his comparison between the em- 
bryos of the higher vertebrates, and the adults of worms, 
crustaceans, spiders, snails, bivalve mollusks, cephalopods, etc. 
On the other hand, many of these comparisons are the same 
as those that are to be found in modern text-books on embry- 
ology; and we may do well to ask ourselves whether these 
may not sound equally absurd a hundred years hence. Why 
do some of. Meckel’s comparisons seem so naive, while others 
have a distinctly modern flavor? In a word, can we justify 
the present belief of some embryologists that the embryos 
of higher forms repeat the adult stages of lower members 
of the same group? It is important to observe that up to 
this time the comparison had always been made between 
the embryo of the higher form and the adult forms of 
existing lower animals. The theory of evolution had, so 
far, had no influence on the interpretation that was later 
given to this resemblance. 
Von Baer opposed the theory of recapitulation that had 
