4 8o CONCLUSIONS 



position of the segment in which it is situated. A matter like 

 this is worth some attention, for it is an example of how 

 principles of general and wide interest can be derived from 

 comparative anatomical studies. 



In sharp contrast to homologous structures are the re- 

 semblances between different and unrelated groups of animals 

 as regards characters which can be proved to have been 

 separately and independently evolved. These resemblances are 

 analogies, and they give rise to the phenomenon of convergence 

 in evolution which is well illustrated by vertebrates. The 

 instances of convergence which might be given are so numerous 

 that only very few need be mentioned here. A good example 

 is the modification of the pentadactyl limb into a paddle, 

 thereby losing its typical appearance and presenting a super- 

 ficial resemblance to the fins of fish. But the interesting thing 

 is that this process has occurred not once but several times, 

 independently, in different groups of Tetrapods : Chelonia, 

 Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosaurs, Thalatto- 

 suchia, Penguins, Cetacea (whales), Carnivora pinnipedia (seals), 

 and Sirenia. The Ichthyosaurs and some of the Cetacea are 

 further interesting in that they have developed median dorsal 

 fins which are superficially very similar to those of fish. The 

 Urodela also have median fins ; but in all these cases, a little 

 study suffices to show that these structures not only differ 

 very much from the fins of fish, but also that they differ 

 between themselves. 



Convergence is also to be found in the case of the elongated 

 and limbless condition of Gymnophiona, certain lizards such 

 as Anguis, Amphisbaena, Scincus, and the snakes. Or again, 

 the fore limb has been modified into a wing independently 

 in Pterosaurs, birds, and bats. The marsupial " mole " 

 Notoryctes is very similar to the true placental moles (Talpa). 



Now it is noteworthy that these cases of convergence are 

 each of them related to a particular mode of life. So the paddle- 

 like modification of the limbs is an adaptation to life in the 

 water, just as wings are adapted to life in the air ; the limbless 

 condition is a form of adaptation to a burrowing habit, while 

 another form of this habit characterises the " moles." It is 



