WHAT WE OWE TO DARWIN 25 



Thus there is convincing completeness in the 

 series uniting various species of the freshwater 

 snails, Paludina and Planorbis, and various types 

 of Ammonites. In the same way the remarkable 

 series of fossil horses, elephants, and crocodiles, 

 are either records of pedigree or conundrums. 



(/) The development of the individual is often 

 in some measure interpretable as a condensed 

 recapitulation of the presumed racial evolution. 

 The individual, as Prof. Milnes Marshall said, 

 climbs up its own genealogical tree. Ontogeny 

 tends to recapitulate phylogeny, especially as 

 regards the stages passed through by a particular 

 organ, such as the brain or the heart. In their 

 early stages there is a remarkable resemblance 

 between the embryos of the higher Vertebrates : 

 they seem, as it were^ to travel for some distance 

 ^loiig the same road before they diverge on their 

 several paths. Gill-slits occur in the development 

 of the^ embryos of reptiles, birds, and mammals, 

 although they have no respiratory significance and 

 are not of any use at all except that one seems to 

 become the Eustachian tube connecting the ear 

 with the back of the mouth. The young tadpole 

 of the frog is fish-like in many details, e.g. as 

 regards the heart and circulation. The very 

 unsymmetrical flat fishes, such as flounder and 

 sole, pass through a syrmnetrical stage. In Fritz 

 Miiller's " Facts for Darwin " the recapitulation 

 idea was applied in detail to Crustaceans, and it 

 seems impossible to understand the often very 

 circuitous development unless it has an historical 

 significance. 



(g) The facts of geographical distribution in 

 past and present suggest the gradual dispersal of 



