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 savs, 

 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 



