256 SECRETS OF ANIMAL LIFE 



mental architecture and mode of development, and 

 is always an indication of blood relationship, either 

 near or remote. The only readily available' dis- 

 cussion of convergence is Professor Arthur Willey's 

 important essay Convergence in Evolution (1911), 

 from which we have taken several illustrations. 



Queerest of queer fishes is the sea-horse, Hippo- 

 campus, often seen in aquaria, which hangs itself 

 from, or supports itself on, seaweeds by means of 

 a prehensile tail, which moves dorsoventrally, not 

 laterally, as in other fishes. It has a rapidly 

 vibrating unpaired fin on its back, and the peculiarity 

 of rolling its large eyes independently of one an- 

 other. Now, it is curious that the far-removed 

 chameleon, which is a quaint arboreal lizard, should 

 show the same sort of prehensile tail as the sea- 

 horse, only more so, and the same independent roll- 

 ing of the eyes. But, as Professor Willey points 

 out, the pipe-fishes, which are related to the sea- 

 horse, but have not prehensile tails, also show inde- 

 pendently rolling eyes. Therefore the association of 

 separately moving eyes and prehensile tail is rather 

 a coincidence. The significance of the mobile eyes 

 is in relation to the sluggish habits and the relative 

 inflexibility of the body in sea-horse and chameleon 

 alike. 



Take another example. With the exception of 

 the American opossums and selvas, all living 

 marsupials (pouched mammals) are natives of 

 Australasia, where, by a geological change, their 

 ancestors were insulated, and thus left to evolve 



