122 KAUL KKNST VON BAKU 



with them in isolated details of structure. The deduction is, 

 however, not borne out by the facts. The embryo of a 

 mammal resembles in many points, at different stages of 

 its development, the adult state of a fish ; it has gill-slits 

 and complete aortic arches, a two-chambered heart, and 

 so on. But at no time does it combine all the essential 

 characters of a fish ; nor has it ever the tail of a fish, nor 

 the fins, uor the shape. Any recapitulation there ma}' be is 

 a recapitulation of single organs, there is never a repetition 

 of the complete organisation of a fish. This is indeed the 

 fundamental criticism of the theory of parallelism; and if it 

 applies even within the limits of the vertebrate phylum, 

 so much the more does it apply to comparisons between 

 embryonic Vertebrates and adult Invertebrates. 



There are also some lesser arguments which might be 

 urged against the theory of parallelism. If the theory 

 were strictly true, no state which is permanent in a 

 higher animal could be passed through by an animal lower 

 in the scale. But birds, which are lower in the scale than 

 mammals, pass through a stage in which they resemble 

 mammals in certain respects much more than they do when 

 adult, for in an embryonic condition they agree with 

 mammals in having no feathers, no air sacs, no pneumatic 

 sacs in the bones, no beak. Their brain also resembles that 

 of mammals more in an earlier stage than it does later. So, 

 too, myriapods and hydrachnids have at birth three pairs of 

 feet, and resemble at this stage adult insects, which form a 

 higher class. 



Again, were the analogy between the development of 

 the individual and the evolution of the Eclicllc tics it res 

 complete, organs and organ-systems ought to develop in the 

 individual in the order in which they appear in the scale of 

 beings. But this is not always the case. In fish the hinder 

 extremity develops only its terminal joint, while in the 

 embryos of higher animals the basal joint is the first to 

 appear. 



Another consequence one would expect to find realised, 

 were the theory <>f parallelism correct, is the late appearance 

 in development of parts which arc confined to the higher 

 animals. In the development of a Vertebrate accordingly 



